Renewed World

At about 47 million years ago, this is the Eocene Epoch of the Paleogene Period. What in the future will become the American state of Wyoming is now a tropical to subtropical savannah, dotted by pieces of more exuberant greenery concentrated around bodies of water. Volcanism is ongoing in this location, occasionally enveloping parts of the biota in grisly deaths of fire and ash. It results from tectonic activity that started during the Late Cretaceous and will still continue for some millions of years ahead, leaving many remarkable traces that last to the present day, like the Devil's Tower geological landmark, formed by polygonal columns of igneous rock (a result of the contractions and subsequent cracks of cooling lava). Since our last journey, continents have retained quite a bit of their positions, though with a few exceptions: Australasia has separated from Antarctica and India has just recently collided with Asia, dramatically influencing the climate as will be soon discussed. North America’s inland sea, which had already receded by the latest of the Cretaceous, has been completely gone for a few million years. South America and Africa continue to drift further apart, while the latter also moves north, towards Europe.

In regards to the world climate, since the end of the Mesozoic, there have been two thermal maximums, with one still to occur. The planet thus continues in a greenhouse, with such high temperatures, a result of elevated levels of carbon dioxide, being likely correlated with tectonic factors, fueling outgassing and the aforementioned volcanic activity. One important site of such factors is the Indian-Asian collision, where the Himalayas are taking shape. This mountain range will, however, eventually take part in an opposite phenomenon: cooling. As the rocks that surfaced from this uplifting weather, great amounts of carbon dioxide will be absorbed and, simultaneously, the collision, while initially promoting more outgassing, will reduce the rate of this phenomenon, lowering the emission of greenhouse gases. Additionally, changes in ocean currents (resulting, for example, from the continued northward movement of Australia) will further cool down other areas, such as Antarctica, kickstarting the start of permanent ice in some regions, though glaciation will not occur uniformly, being subjects to bouts of occasional warming.

Let us now focus, nevertheless, on the location of this tale, occurring in a piece of Western North America distant from the shore. Walking close by to a lake in search of food is the large bird Diatryma, reaching up to 2 meters in height. It is predominantly a seed eater, collecting acorns fallen from the oak trees (Quercus) spread across this environment, eudicots that still are immensely important in the composition of modern Northern Hemisphere flora. Both the leaves and fruits (the acorns) of oaks are poisonous to various animals, being rich in different compounds such as tannins, which interact with proteins and may alter their properties, being employed by plants as defensive means. Even so, they serve as sustenance to many creatures besides the Diatryma, harboring many leaf-consuming insects that feed numerous other birds. The influence of tannins can also be observed in the close-by lake: its waters have a brownish color resulting from the runoff of tannins from neighboring decaying vegetation.

Underground, these trees can even associate with truffles, filamentous subterraneous fungi that form intimate connections with the roots of its host plant (such connections extend to the cellular level, with fungal cells penetrating vegetal ones). The fungus acquires nutrients from the surrounding soil that the phototroph needs and the photosynthesizer provides the fellow eukaryote with sugars derived from its photosynthetic activity. In certain times of year, the truffle itself is formed, being the tuber-shaped fruiting body of the fungus, containing spores that, to be disseminated, need to be ingested by specific animals, attracted to their meals by pungent odors that emanate from them. Yet another tree, in this case, a monocot, is the palm Sabalites, which, already existing by the Late Cretaceous, has quite a widespread distribution, growing in areas warm and wet enough for its survival, such as here. Like the distantly related ferns, it possesses rhizomes, sending out roots that spread in a radial pattern.

Returning to the Diatryma, it is a basal genus of the Galliformes order (including chickens, turkeys, peafowls, and pheasants for instance), actually originating from Europe (where also exists the very similar genus Gastornis, with Diatryma being heavier and stouter), having arrived in North America earlier during this epoch, being here represented by the species D. gigantea. To aid in its digestion, it consumes pebbles, like its long gone relative, Elaphrosaurus, and, like many other galliforms, this coelurosaurian theropod presents pronounced bare parts, serving an important function not only in sexual display but also in thermoregulation, especially in fairly warm environments such as the one it finds itself in and additionally due to its large body size. Though this individual is currently alone, Diatryma tend to be gregarious. Females roam territories containing various males, which are the only ones to invest in parenting, with their precocial chicks following them around for quite some time, later dispersing.

A bit distant, another dinosaur can be seen, at least what is left of its remains: the fossilized skull of a Torosaurus, a chasmosaurine ceratopsid (discussed in more detail in the last chapter). Its impressive and large head, resting on the lake’s bank after being brought here by powerful rains in a block of debris that has since vanished, is a testament to a world renewed, but also one in continuum with the past, as always occurs. New organisms, after all (except during the primordial abiogenesis very early in Earth’s history), only arise from existing ones, with life thus being an uninterrupted cascade of lifeforms. Fossilization in particular is a process that lets clear the planet all Earthly organisms have inhibited is the same one, truly putting such continuum quite in evidence. In regards to the process, it is an incredible one at that and depends on the dead being quickly buried by sediment, as to avoid its scavenging and decomposition. Burial in areas lacking many decomposers, such as in anoxic ones, also facilitates preservation. Once the body is protected from such degrading agents, time takes its course and gradually, if the remains stay, they become mineralized, turning into objects like this Torosaurus skull.

Coming back to the living, two genera of ungulates (popularly known as the “hoofed mammals”, being placentals, as will be all members of Mammalia seen during this tale) can be seen, differing drastically in their modes of life. First are many Orohippus, very small equids belonging, like the rest of their family, to the order Perissodactyla (called the “odd-toed ungulates” and including, apart from equids, tapirs and rhinoceroses for example). Though bearing not much resemblance to the later and more familiar equids, characterized by their larger size and single hoof on which they sustain themselves, this genus of shy herbivores is possibly the first of its family to exhibit minor changes to hindlimb articulations that, nevertheless, prove important in joint stabilization, a feature that will be significant in the highly cursorial members of Equidae yet to evolve.

The other ungulate is a Mesonyx, a predator growing to a bit more than 1.5 meters in length thanks in part to its long tail. With an uncertain classification, it varies from a basal position to one quite deep inside the main another ungulate group: Artiodactyla (called the “even-toed ungulates”, including cetaceans, ruminants, swine, hippopotamuses, and others). If indeed an artiodactyl, Mesonyx would stand close to Cetacea, branching off way before they took to the waters. This individual, a male, is currently sniffing the ground, looking for the scent of receptive females. Normally, however, it relies more on other sensory cues, not having quite a very developed smell. Despite distant from hunting intentions, the Orohippus try to stay a good distance from it, alert to any possible moves from their carnivorous cousin, capable of efficiently running down most prey items. Like the aforementioned Diatryma, Mesonyx also has a distribution extending beyond North America, occurring in Asia, where it plays quite a similar ecological role.

The underbrush, composed by constantly seen plants such as ferns, is nevertheless dominated, like the rest of the flora, by angiosperms, such as varied herbaceous forms. Apart from them, vines of the eudicot family Cucurbitaceae cover the forest floor, sending various shoots and tendrils, with large, lobed leaves. Occurring mostly in tropical regions, they are still widespread in the present, represented by well-known forms like watermelons, cucumbers, pumpkins, etc. And in the present, just like during this tale, they are subject to many dangers, dangers which we, as their cultivators, fight fiercely against. One of these dangers comes in the form of the squash bug (Anasa tristis), a hemipteran native and endemic to this landmass, North America, without a doubt sharing a long history of interaction with the cucurbits. 

This is especially the case because these are far from defenseless plants: they produce extremely bitter and dangerous toxins known as cucurbitacins. The bitterness is no coincidence, as it serves as a warning to any herbivores foolish enough to continue eating. However, with the squash bugs, this is no problem. These gray and almost shield-shaped insects insert their proboscises in the leaves and stems of their victims unworryingly, since they are completely resistant. Not only do they constitute a problem by themselves, but they also transmit disease in the form of a pathogen that, amazingly, can also affect humans and other animals. Such pathogen is the flagellate, rod-shaped bacterium Serratia marcescens, being widespread in all sort of environments and belonging to the class Gammaproteobacteria. 

Various strains are characterized by their red coloration (they are often seen as reddish slime growing in bathrooms), a result of a pigment that goes by the name prodigiosin, with several biological properties, including antibacterial and antifungal ones, perhaps being used by the S. marcescens to eliminate competition. In the case of the Cucurbitaceae, these microbes establish colonies in their phloems and gradually interrupt sap transport. With this, the roots stop receiving sugars and soon rot. On top, leaves start turning yellow, later withering and dying. Young plants, more susceptible, have a more rapid progression of the disease and can collapse completely in just a few days. Either way, the cucurbits of this underbrush are healthy and bright green. Moving through them with an undulating movement, slowly throwing its legs side to side, is a varanoid lizard, more specifically a Saniwa. Like other lizards, it is a squamate, part of the larger Lepidosauromorpha, the main other reptile grouping besides the more usually seen Archosauromorpha.

Interested in getting a drink, the amniote calmly approaches the lake, occasionally flicking its forked tongue out. Such act, perhaps superficially coming off as just a quirk, is, as a matter of fact, integral to the squamate lifestyle, being a way for these creatures to pick up chemical cues (like pheromones) from their surroundings, essentially constituting a sixth sense, which is combined with the other ones to grant such reptiles deep sensory insights. Saniwa specifically, around 1.3 meters in length, has even a more peculiar sensorial trait: instead of only one, it possesses two parietal eyes, a condition nowadays only seen in the lampreys, jawless vertebrates. This photosensitive structure (previously mentioned during our trip to the Permian) likely serves its owner a navigating function, helping it to orient itself according to the celestial bodies. After taking its drink, the lizard, instead of returning to the underbrush from where it came, takes to the water, where it hopes to find some prey or at least arrive in some part of the forest floor with more plentiful food opportunities.

As it dives into the lake, small ray-finned fish of the Clupeiformes order disperse, frantically swimming away from their osteichthyan relative. Even though members of this order, composed of herrings and anchovies, normally are inhabitants of salt water, freshwater residents like these also exist. Many other actinopterygians also swim in these stained waters. The predatory gars, fairly elongated with lengthy snouts filled with sharp teeth (like the Richmondichthys of a previous tale, they display ganoine scales), are some examples, exhibiting quite a significant dietary shift with age: younglings lack the characteristic long jaws of piscivorous adults, having a more robust feeding apparatus appropriate for their arthropod meals. More peaceful, but in no way less impressive are the paddlefish, possessing quite a longer upper jaw that, filled with electroreceptors, serves a sensory function, apart from also helping direct zooplankton and other fishes into their wide gaping mouths. Despite being bony fishes, most of their skeleton actually is cartilaginous and, at present, only one species of paddlefish remains, still in North America, with a larger species from China having become recently extinct.

Crustaceans too are abundant, counting with the tiny ubiquitous ostracods (first cited in our visit to the Jurassic) and larger forms such as crayfish, part of the group Astacidae together with lobsters. Despite a superficial resemblance, crayfish inhabit freshwater habitats like this one and probably diverged from their marine relatives during the end of the Paleozoic and start of the Mesozoic. Taking to continental waters, the crayfish would disperse thanks to Pangaea, with the supercontinent’s breakup leaving the invertebrate’s populations stranded, generating two superfamilies of crayfishes, a southern and a northern one. With a great variety of species, these arthropods possess a wide variety of habits, some species being burrowers and others occupants of open waters. Their diet is also subject to variation, ranging from carnivory and herbivory to more generalized omnivory/detritivory, with their feeding actions being important in shaping their home areas by shifting through sediment (similar to how worms, at the Phanerozoic’s beginning, radically modified marine substrate, as discussed in our first tale). Their claws, despite what one might initially believe, are not so much used for food manipulation as for sexual matters, both for mate disputes and for copulation itself.

Either way, though the water is brimming with macroscopic life, it is even fuller of microscopic organisms. Protozoans, an informal name for various unicellular eukaryotes, are some of the largest of such lifeforms. Even between them, there is tremendous variation in dimension, but also in lifestyle. Some are encountered mostly at the substrate, a habitat which, often counting with very low amounts of oxygen, provides a safe haven for obligate anaerobes (for which oxygen is toxic). They do not live alone and many sport endosymbiotic methanogens, non-eukaryotic Archaea first mentioned here. Others, however, live up in the water column. Usually the most abundant protozoans are ciliates, characterized by their numerous cilia and by an invagination of the plasma membrane that serves as a site for the acquisition of food. Incredibly diverse, they occupy niches in all levels of the freshwater body: a few are photosynthetic (counting with endosymbionts of oxygenic or anoxygenic photosynthesis, also discussed in more detail in the entry linked above) and others use their cilia to filter-feed or either actively catch their prey, with their foodstuffs varying wildly, from bacteria to algae, to other protozoans.

Amoeba, in contrast, possess an undefined body shape, mostly inhabiting solid surfaces where they move by cellular projections known as pseudopods. Unlike the ciliates, food is not acquired through a specific point of entry, being captured by a process of engulfment (called phagocytosis) using the just cited pseudopods: their prey too generally consists of bacteria and algae, despite the fact larger amoeba can consume more sizeable particles. Phagocytosis is such an important process for amoeba that some DNA-based viruses have used this physiological necessity for their parasitism. They are giant viruses (on the same scale as small bacteria and some archaea), using amoeba to replicate themselves, their large dimensions being a result of the fact that the phagocytic process is dependent on size (particles can be neither too small nor too large) and their relatively large genomes springing from various duplications and events of horizontal gene transfer with their own hosts, to the point that the majority of their genetic material is host-derived (this is in great concordance with the "chimeric-origin hypothesis" for viral evolution, better discussed here). 

Despite taking on similar hosts, these parasites display a great variety of shapes. For example, the Mimivirus of the modern day is characterized by a polygonal capsid covered in long fibers (consisting of a possible attractant for amoeba), with part of the capsid actually having a starfish-shaped elevation, a structure probably acting in the delivery of DNA to the host cell. Others, like the extant Tupanvirus, have even more peculiar morphologies, with such genus having a very similar capsid to the one of Mimivirus, but differing by counting with a fairly long cylindrical tail. Besides these structural properties, the Tupanvirus also promotes the aggregation of both infected and uninfected amoeba, guaranteeing that, when the infected cells rupture, the newly released viruses will immediately encounter suitable hosts. Quite curiously, the replication and assembly machinery formed by these viruses in the cytoplasm of their protozoan hosts can be utilized by much smaller viruses, known as virophages. These much tinier parasites negatively impact the reproduction of the larger ones, increasing the survival chances of the amoeba unfortunate enough to become infected. Distancing ourselves from viruses, there are also flagellate protozoans, coming from a wide variety of groupings that, instead of cilia or cellular projections, move employing flagella (structures present in the protozoan parasite of tyrannosaurids seen in the last chapter), which, nonetheless, can also be used in feeding, either via filtering floating bacteria or by snatching them. Similarly to the ciliates and unlike the amoeba, such flagellated organisms have a more widespread distribution along their environment, some being photosynthetic too (like the euglenids also mentioned in the first entry).

Focusing on the prey, bacteria are even more abundant and numerous, constituting a grand portion of the base of the lake’s food web. Very small and frequent are the ones belonging to the phylum Actinobacteria (the pathogenic genus Mycobacterium, responsible for causing diseases such as tuberculosis and leprosy, is part of this phylum), commonly encountered in a manner directly proportional to oxygen concentration, being most abundant closer to the water surface. Due to their aforementioned small size and specific cell wall structure, counting with an additional proteinaceous covering apart from the peptidoglycan layer, these prokaryotes are somewhat resistant to predation by many protozoans, but not by all. They also synthesize rhodopsins, proteins that bind to retinal (derived from vitamin A) and which enable them to possibly engage in anoxygenic photosynthesis. Animals also count with rhodopsins, indispensable for the function of photoreceptor cells, but these are not homologous to the microbial rhodopsins, having convergently evolved.

Another bacterial phylum is Bacteroidetes. These are important decomposers of organic material and, thus, are associated with periods of high nutrient load, increasing in numbers during blooms of phytoplankton for instance. Some, when in conditions of increased predation, may adopt a filamentous shape (in which cells do not detach after dividing), hampering their ingestion by hunters. Cyanobacteria are also found abundantly in lakes such as these. Mentioned and encountered quite some times before, these bacteria are oxygenic photosynthesizers, with a few even partaking in nitrogen fixation, being fundamental members of the just before-cited phytoplankton. Bacteria of the class Alphaproteobacteria (mitochondria arose from this grouping) are also important components of the bacterial biota, though not quite as copious. Showing quite some resistance to predation by protozoans, these usually heavily interact with other organisms, acting either as symbionts or parasites, affecting, for example, cyanobacterial growth. The class Betaproteobacteria is, like the previous one, a member of the more inclusive phylum Proteobacteria, but its members are quite more abundant in bodies of freshwater, thriving during periods of increased nutrients and exhibiting a susceptibility to predation inversely correlated with size, with smaller forms being less predated upon (in a similar fashion to what occurs in the previously discussed actinobacteria).

A few bacteria, like the giant viruses, have also adapted favourably to the phagocytic nature of amoebas, shifting from prey to opportunists, with implications for their pathogenic effects in animals, including ourselves. One such possible species is the extant Listeria monocytogenes, a rod-shaped bacterium of the phylum Bacillota. Commonly encountered in the environment, where it acts as a decomposer, L. monocytogenes can occasionally act as a pathogen, finding its way inside the digestive tract of hosts through contaminated foodstuffs. Counting with four to six flagella distributed all around its cylindrical body (it is fundamental to stress that bacterial flagella are very different from eukaryotic flagella, despite serving similar functions and sharing a similar, thread-like appearance, both evolved independently, with bacterial flagella rotating and eukaryotic flagella, homologous with the also eukaryotic cilia, beating and being much bigger, while the non-eukaryotic archaeal flagella, called an archaella, too rotates, but shares an independent origin from the bacterial and eukaryotic flagella as well), it utilizes its movement to invade epithelial cells lining the gut lumen and thus cross the intestinal barrier. 

Once inside the body proper, L. monocytogenes, now without flagella, enters in contact with various other cells, including those of the immune system (which will soon be discussed in much more detail), some of which, called macrophages, exhibit an amoeboid shape and an equivalent phagocytic behavior. After internalized by these cells, the bacterium utilizes different proteins to escape from a digestive vacuole and proceeds not only to divide, but to polymerize specific cytoskeletal filaments in the cell's interior, forming for itself an intracellular propeller that not only moves it inside the parasitized cell, but also creates a cellular projection that allows it to colonize neighboring cells by forming bridges, with it all the while being protected from the more hostile external bodily habitat.

Anyhow, many other bacterial phyla can also be found in this not so large body of water, with this being just a subtle peek at the amazing microbial diversity. That being said, species of unicellular lifeforms, different from what more commonly occurs with multicellular beings, are normally ubiquitous throughout the world, not showing restricted geographical distributions. This is in part because they have such high numbers that, by pure chance, dispersal is achieved, occurring by abiotic factors (like wind and water) or biotic ones (microbes getting transported by animals for instance). The fact that many are able to generate cysts or spores, forms more resistant to environmental stresses, also increase their rates of environment colonization. This means that even though an incredibly high number of species may exist in a fairly small space such as this lake, a similar number of species will also be present if counting all lakes in the world. The proportion of species is also usually maintained, with more common species locally being more common globally and vice-versa.

Transitioning from the microscopic to the largest animals of this location we have the dinoceratans, represented by the genera Uintatherium and Tetheopsis, the former achieving around 4 meters in length and the latter being a bit larger. With not quite a clear phylogeny, these titanic herbivores are possibly most closely related to a group of South American ungulates called the Xenungulata, which, including other South American ungulate groups, are distinct from either the artiodactyls or perissodactyls, being collectively known as the South American Native Ungulates (examples will be seen in the next tale), though such hoofed mammals likely do not constitute a monophyletic aggregate. If such a relationship between the Dinocerata and Xenungulata is indeed the case, it is just another of the reflections of the mammal dispersal event originating from North America around the very end of the Cretaceous that sowed these ungulates in its isolated southern neighbor. But this will not be the end of faunal interchanges regarding South America and another one will take place in a far more significant manner millions of years in the future.

Despite a distant relation, these two dinoceratans share with the extant giraffes (part of the Artiodactyla) two curious characteristics: skin-covered bony projections called ossicones and long, mobile tongues, apt for manipulating vegetable matter. A flexible upper lip is another useful trait for handling plants and their large canines, a sexually dimorphic trait (with males having longer teeth), are sometimes used for pulling soft phototrophs from the water. The Uintatherium pair that has come to the lake for a drink and the resting Tetheopsis, nonchalantly looking at its surroundings, content to just cool off in the rich waters while occasionally flipping its ears to ward off an inconvenient insect, convincingly pass the idea of gentle giants. And, for the most part, they truly are. With no predators even coming close of posing a threat to them after achieving adult dimensions, life is tranquil. That, however, suddenly changes during mating time. Testosterone-charged males become aggressive and engage in violent fights with their peers, exchanging blows and lacerating each other with their prominent teeth.

In regards to reproduction, an intriguing phenomenon has become more common over the last thousands of years, as the older Uintatherium has come into increasing contact with the more recently evolved Tetheopsis due to the expansion of the savannahs, which has made the two ungulates occupy increasingly shared spaces, since both, being primarily browsers, have a special affinity for the lower hanging branches of trees. The two are very closely related and many Uintatherium females have selected male Tetheopsis as mates, mostly because these, being larger than their relatives, are able to more easily overpower and deny their interspecific rivals female access. Even though some hybrids are born of these matings, most do not lead to offspring and even those that do, such offspring are usually infertile, essentially meaning the Uintatherium are being gradually absorbed into the Tetheopsis population. 

Organisms that hybridize even more readily than animals are plants, with it being suspected that up to 25% of vascular plants undergo such process. It is believed that one of the reasons vegetals can generate more hybrids is their more plastic development, allowing for greater genetic changes without compromise to the individual. Plants are, after all, modular organisms, lifeforms that vary significantly in their final morphology, greatly influenced by the environment (trees of the same species, for example, do not have a fixed number of branches and can considerably vary in their shape). Fungi, sponges, and corals are other examples of modular organisms. Most animals, though, belong to a group known as unitary organisms, which have their bodies predetermined and fixed, consequently being less open to such genetic changes, ones that usually have disruptive effects in their case.

Like many other organisms, the Uintatherium and Tetheopsis are subject to a wide array of parasites, including small, blood-sucking arachnids: ticks (which, like the crayfish, likely evolved all the way back in the Paleozoic, more specifically around the Carboniferous-Permian transition). These are specifically of the family Ixodidae (despite their probable Paleozoic origin, these arthropods only diverged into separate families during the Cretaceous), characterized by the presence of a hardened shield and fairly proeminent mouthparts (ticks of the family Argasidae have a leathery exoskeleton, with their mouthparts hid under their bodies). As such mouthparts pierce the hide of the unfortunate mammalian host and make their way to the vessels lying below, many compounds recognized as foreign by cells of the victim are introduced, while the female ticks, along several days, baloon in size, the males hardly enlarging. Vertebrates, apart form innate immunity, have quite sophisticated mechanisms that allow for selective and more efficient responses to such inserted substances, these being undertaken by the adaptive immune system.

T and B lymphocytes (also known as T and B cells) are the main components of such system, both expressing recombination activating genes (RAGs), responsible for encoding an enzyme that allows the recombination of T-cell receptor genes and antibody genes (antibodies are synthesized by the B lymphocytes). The end product of these recombinations essentially means these cells produce an insanely high number of unique receptors or antibodies, each one capable of binding to a particular material (called antigen), including the ones of their own organism. As such, they are selected to only elicit responses against foreign antigens.

In the case of T lymphocytes, this occurs in the thymus, an organ fairly conserved among gnathostomes (jawed vertebrates) where thymic nurse cells individually internalize tens of T cells. The nurse cells have as their main function negatively selecting lymphocytes reactive against self-antigens, inducing, for instance, their apoptosis (a form of programmed cell death). It is this form of control that helps inhibit autoimmune diseases (such as the alopecia affecting the Trucidocynodon seen in the Triassic) and guarantees that only foreign substances, like the ones being currently introduced by the tick, are subject to retaliation. However, not all of these substances result in immune activity. Food, for instance, normally promotes no reaction due to a process known as oral tolerance, occurring in great part due to T regulatory cells, specific T lymphocytes that, generally taking on a suppressor role, are also important in inhibiting autoreactive T cells not eliminated in the thymus.

Returning to the inoculation of foreign compounds by the tick, these, either being living organisms or simply molecules, will initially be dealt with by the cells of innate immunity. They, however, will be key in promoting an adaptive response, a feat made possible by antigen presentation: a process in which any cell, but more specifically macrophages, monocytes, dendritic cells, and B lymphocytes (the action of B cells in this regard will be explained in further detail), which are specialized for such function and, consequently, are known as professional antigen-presenting cells, essentially present antigens processed either from the extracellular space or from inside their own bodies to T lymphocytes. The first is done only by the mentioned cells (with the antigens being presented on major histocompatibility complex II molecules, called MHC II), but the latter is undertaken by virtually all other cells (with the antigens being presented on major histocompatibility complex I molecules, called MHC I) and, sometimes, antigen-presenting cells can present antigens not derived from themselves on MHC I molecules, an event known as cross-presentation.

In regards to the MHC molecules, they are highly polymorphic proteins expressed at the membrane of cells, existing in a wide array of varieties inside the same population. This elevated polymorphism ensures that as many antigens as possible are presented (some MHC molecules may not bind as well to an antigen as some other MHC molecules and vice-versa: many different MHC molecules promote a higher probability of successful binding and, therefore, presentation) and that antigens are indeed presented (some pathogens have mechanisms for lowering the presentation of certain antigens by MHC molecules: the existence of various distinct MHC molecules makes it more likely such downregulation will not be always successful).

Either way, the professional antigen-presenting cells need not only to present antigens to the T lymphocytes, since these, in order to be activated, require other stimuli: additional surface molecules other than the MHC and secreted proteins with great signaling action called cytokines. If such activation occurs, T lymphocytes will suffer differentiation and proliferation, eventually aiding the innate immune system in dealing with the foreign compounds introduced in the first place. 

B lymphocytes, though, are intriguing because they not only activate T cells but are, in turn, also activated by them, a reciprocal interaction. For this activation of B cells to occur, they, like their T counterparts, need to be presented with an antigen: this occurs via the antibodies they produce, functioning (as said before) similarly to the T-cell receptors and, thus, also having the capability of binding to specific antigens. Like happens between T lymphocytes and the antigen presenters, other coestimuli are needed for B cell differentiation and proliferation and these are also alike, being surface proteins and the cited cytokines. Once activated by the T cells, some B lymphocytes become plasmacytes, cells that secrete large amounts of antibodies (different from the antibodies found in the membranes of non-activated B cells), proteins that may also aid in the action of the innate immune system or even directly neutralize the foreign compounds, be they pathogens or toxins for instance.

Returning to protozoans once again and exemplifying the disease-transmitting potential of ticks, mammals, as well as birds, parasitized by these arachnids can become the victims of yet another parasite: apicomplexans (first mentioned in the first entry as well) of the order Piroplasmida. Like other members of the phylum Apicomplexa, the piroplasmids are highly polarized cells, possessing, at one of their extremities, an apical complex (probably developed from the flagella of the common ancestor of apicomplexans and dinoflagellates, with these two phyla forming, together with the aforementioned ciliates, the superphylum Alveolata, marked by the presence of sacs under the cellular membrane, sacs that, associated with proteins, confer structural integrity), a conglomerate of cytoskeletal components and secretory organelles essential, in this case, for the invasion of the red blood cells of the mammalian or avian host. Such complex in piroplasmids is quite reduced, but still of great importance. Once inside red blood cells, they feed off the hemoglobin and multiply asexually until the cell bursts, getting released into the bloodstream and infecting more. Some forms, if eventually ingested by ticks, will reproduce sexually in the invertebrate’s gut, then differentiate into other forms that will subsequently become infective, ready to start the cycle once again.

Back once more to the macroscopic organisms, but to the smaller ones in this region: the insect Chimaeroblattina, more specifically a cockroach, part of the order Blattodea, which also includes other hemimetabolans such as termites. Dissimilarly to other cockroaches, this one is not flattened, being, in all actuality, a mimic of hymenopterans, more specifically bees. By passing as a more threatening arthropod, it gets rid of many potential dangers, taking on a less stressful existence, peacefully consuming the very abundant plants of this area.

Other much less conspicuous but far from less important invertebrates lie in the soil: earthworms. Annelid worms, they act, like the crayfish, as essential ecosystem engineers, aiding in soil turnover and decomposition, the products of which may end up in water bodies, fueling the growth of some of the mentioned bacteria. During rain, these normally hidden soft creatures emerge in large numbers from the ground, drowning some parts of the forest floor in pale pink squiggling forms. While it has been suggested that such behavior is a result of the flooding of their holes, such is not the case. Earthworms breathe directly through their skin, a process dependent on moisture (they have a mucus covering important for respiration for instance) and unaffected by a larger degree of surrounding water, possessing, unlike what happens in arthropods, not only a circulatory system but a closed one, counting, like vertebrates, with hemoglobin as a respiratory pigment (but it is extracellular, unbound to cells like happens in the former). If that is the case, what accounts for their dramatic eruption? Apparently, moisture is still part of the answer, but working in the opposite manner: rain leaves the surface more hospitable, allowing them to transverse it more easily and so they will, perhaps on the lookout for better grounds or mates. However, there is another possible reason for such occurrence. As the raindrops fall on the ground, they produce vibrations that the earthworks mistake for predators, scaring them away from their buried positions.

Examples of animals that take advantage of this are Aletornis, here seen forming a pair before the resting dinoceratans. Completely dwarfed by the former, they move attentively, ever watchful of their setting, restlessly bobbing their small heads. Belonging to Gruiformes (an order of birds that includes cranes and rails), they are adapted to leading an amphibious lifestyle, with one having just caught a Viviparus, an aquatic snail. Another attentive observer is the Boavus watching their every move, flicking its long, forked tongue from time to time while staying almost completely immobile. A snake, it is also a squamate, sharing much of the sensorial capabilities with the preceding Saniwa. Ranging from 1 to 2 meters long, it is a fairly small boa (especially when compared to representatives of Boidae like the gigantic Titanoboa, more than 10 meters in length, from the South America of earlier in the Paleogene, or to the modern-day genus Eunectes, growing to some 8 meters from snout to tail), but it will have no difficulty swallowing the Aletornis whole, since, like other representatives of its family, its jaws are quite flexible, with unconnected bones that allow for a great amount of opening.

The dinosaur couple is oblivious to their reptilian relative slowly slithering closer from behind, exerting its patience to a maximum until it achieves the ideal distance. It has been approaching them for quite some time now and it has finally arrived at striking distance: the time is now. In the blink of an eye, it lounges forward. The small birds shriek in terror, dropping the still alive Viviparus back into the water, but making their escape. The Tetheopsis close by looks uninterested at the commotion, never changing its placid expression. As fast as it went, the snake returns to a neutral position, staying put to wait for more unsuspecting avian victims.

Another animal perhaps engaged in hunting is the hyaenodont Sinopa, at some 1.2 meters in length. It stands up by supporting itself against a tree, looking up curiously at a Vulpavus, a carnivoraform, part of a group encompassing but not restricted to the extant Carnivora order (with representatives such as bears, felids, canids, mustelids, pinnipeds, among many others). Unlike many of its carnivoran relatives, marked by a highly carnivorous diet, Vulpavus is omnivorous, consuming a more generalized array of foodstuffs, including many invertebrates. An apt climber, it can secure meals from basically anywhere in the forest and the climbing ability also offers considerable leeway against other, more grounded predators possibly seeking it as a meal. This could be the case with the aforementioned Sinopa, though it may be only curious, peeking at the Vulpavus with what can be interest or frustration, occasionally emitting bark-like sounds at the unimpressed climber. Like others of the order Hyaenodonta, it is truly carnivorous, though that does not make it closer to the carnivorans than the Vulpavus. It, like fellow hyaenodonts, is, instead, part of the more inclusive Ferae, Hyaenodonta being a parallel branch to the Carnivoraformes and the larger Carnivoramorpha that contains it. As is the case with Mesonyx, Sinopa also has an Asian distribution, mirroring its order as a whole, present in Eurasia, Africa, and North America, lacking from the island continents of South America and Oceania.

Either way, it tries to climb, though cumbersomely and with not much success, the trunk of the Lauraceae tree, a family of tropical eudicots with various well-known examples (such as laurel, avocado, and cinnamon) and with many aromatic members, a result of the production of essential oils, which display a wide manner of functions, from defense (even against bacteria) to sending signals to pollinators. One specific genus of Lauraceae is Cinnamomum, popularly known as cinnamon, which takes the form of shrubs and small trees (10 to 15 meters tall), being also distributed in this forest, becoming increasingly rare as one moves into the savannah. Its bark is also full of essential oils, justifying our use of it as spice in the present. One more eudicot tree is the iconic maple (Acer), too coming in the shape, like cinnamon, of shrubs. In the modern day, it still concentrates in the Northern Hemisphere, being found in regions of temperate to tropical climate still in North America as well as in Eurasia and North Africa, with some species producing a very sweet sap (this is the sap which goes through phloem, as explained here), the basis of maple syrup. Towering over all other trees, found deeper within this Eocene forest, are Sequoia, a genus, like the previous two, still living, though with only one species (Sequoia sempervirens), nowadays endemic to the American West Coast, exhibiting thicker leaves and larger cones than the older representative. During this time, though, it is far-reaching, being, like previously seen gymnosperms, part of the Cupressaceae family. The humidity of this environment is essential for the Sequoia, which, in a manner similar to its extant relative, absorbs a considerable portion of water through its leaves from the neighboring air.

Other members of the maple’s family, Sapindaceae (which counts with other familiar representatives, such as guarana and lychee), can be found here. On one blossoming individual, marked by numerous delicate white flowers, hang two primates of the genus Notharctus. They are basal members of the Primates order, inside a grouping known as the adapiforms, characterized by their larger bodies and herbivorous diets, consuming many leaves and fruits. In terms of behavior, they form large bands headed by a dominant male, which monopolizes breeding rights. These two, one of them having just observed the strike of the Boavus and the other just occupied with eating, are females that have wandered a bit farther from the rest of their band, characterized by their bland color, particularly when compared to the males, much more vivid and dotted with striking patterns.

Talking about color, various primates are unique among other mammals by having enhanced color vision, a feat made possible by the reorganization of their retina and reacquisition of a rhodopsin (mentioned earlier in this chapter). Most vertebrates, except for mammals and amphibians, count with four cone rhodopsins (cones are retina cells that are responsive in bright light, coming in different varieties, each containing a particular rhodopsin that permits the distinguishment of colors, whereas rods are sensitive to much lower light concentration, but only coming in one variety, with one rhodopsin, not allowing for colored vision, though both photosensitive cell types act in tandem, extrapolating these individual functions). Ancestral mammals, on the other hand, had only three, and, along evolution, members of Mammalia independently lost them, rendering most mammals with only two remaining rhodopsins. This loss was potentially correlated with nocturnal habits, with mutated, non-functional rhodopsins not only not being selected against, but possibly even increasing visual acuity, explaining the general occurrence of such degeneration, though multiple other factors were likely at play too. Many primates, however, like the adapiforms themselves, are diurnal and consume foodstuffs, such as fruit and leaves, that give telltale clues of their edibility based on color, apart from maybe helping in harvesting ideal meals when foraging. This has meant that several primates redeveloped three rhodopsin genes (including the Notharctus and its close relatives), returning to the ancestral mammalian number, though some species (many nocturnal) retained the dichromatic vision of their ancestors.

Higher in the Sapindaceae tree, two tetrachromatic vertebrates, birds of the genus Zygodactylus (also found in Europe), desperately try to protect their chicks from a horde of hunting Titanomyrma ants. They fly agitated around the branch, eager to strike the hymenopterans, but fearful of landing and getting attacked themselves, for these ants, as part of the subfamily Formicinae, may lack stings, but can spray irritating formic acid at their enemies. Besides, the large size makes their bites painful. As such, the amniotes turn increasingly nervous as they hear their progeny’s calls of distress, with the couple being restricted to fly-bys where they launch the arthropods to the ground using their beaks and feet. Regarding their feet, they exhibit a condition called zygodactyly, in which two of their toes are positioned backward, something also seen in birds of the order Psittaciformes (parrots) and which may be ancestral to both these and the Passeriformes (the largest bird order, ranging from crows to finches to ovenbirds among many more), the sister clade to the family Zygodactylidae, of which Zygodactylus, as the names clearly suggest, is a member. In the Passeriformes though, zygodactyly as a possible ancestral condition was lost and gave way to their characteristic trait: anisodactyly, marked by three forward-facing digits and only one directed backward.

Whatever the bird’s toe orientations may be, this is of no importance to the Titanomyrma, giant ants that, with queens up to 5 centimeters in length, also encompass Europe in their distribution, with the European species being even larger than this American dweller, T. lubei. Such distribution is the case for various animals listed due to the existence of a land bridge between the two continents, with migration routes going through the unglaciated Arctic and areas of modern-day Iceland and Great Britain. There is also a connection between North America and Asia through the Bering land bridge, also allowing for shared fauna, as seen earlier. Though usually targeting invertebrates, these are members of a particularly hungry colony, recently displaced due to the aggressive action of other Titanomyrma. Despite their determination and large size, the birds’ eagerness to defend their offspring will likely mean a failure of their plans, though more and more pheromones are being released by the workers at the frontline, culminating in a growing line of ants climbing this Sapindaceae’s trunk. It is truly a mystery which creature shall come out on top.

Down on the ground is a Patriofelis, dragging along a Chisternon turtle recently captured from the lake. The predator is the largest carnivorous mammal in this environment, belonging to the order Oxyaenodonta, which, like Hyaenodonta, is also a part of Ferae, too constituting a side branch to the Carnivoramorpha. Despite this somewhat equivalent phylogenetic placement, the two orders differ greatly in other aspects. Oxyaenodonts are generally stockier, possessing less elongated and broader faces, apart from shorter legs. For Patriofelis in particular, achieving close to 2 meters long, this means it is an ambush predator, heavily distanced from the cursorial lifestyle of the previously seen Mesonyx. It is, nevertheless, a powerful creature, ensuring the victims that come close enough to it do not escape. This was the case with this turtle. It stayed resting at the lake’s bank too long. When it finally decided to escape, it already was too late. The oxyaendont’s wide paws, apt at transversing this location’s waterlogged terrain, had already firmly gripped it and death would come shortly after as the jaws of its attacker closed in around its head. Its left eye then saw with helplessness the approaching darkness, accompanied by a warm and humid air that progressively involved it, until it was all gone. Slowly the Patriofelis would turn around, carrying the limp body of its prey back to land, streaks of blood forming in the water as the bodily fluid trickled down from the dead reptile’s injuries. The Chisternon is part of the Baenidae family, freshwater turtles that originated in the Early Cretaceous, having radiated into several species since then. Even so, this genus, along with the related Baena, represents the last members of their family, acquiring considerable dimensions at the time of their twilight.

As the hours go by, a literal twilight envelops the forest, as many new sounds, like the croaking of frogs anxious to mate, take over the backdrop. Not only those amphibians are eager to take on reproductive matters though. Far from the lake and deeper among the trees a quick succession of sounds signals an intense occurrence. Two male Uintatherium are squaring off, filling the night air with snorts and grunts that contrast greatly with the soft whistles the dinoceratan calves use to communicate with their mothers. As the two giants fight on, stomping their columnar feet and shoving their heads, they bring disarray to their surroundings. The den of a Sinopa couple is on the fighters' track. As the battling behemoths approach, the two hyaenodont adults growl and bark to no avail, while their pups, normally playful and excitable, whine in terror behind their backs. Seconds pass and the bewildered ungulates continue on, uninterruptedly pushing each other until they already are at the home of the small carnivores, which, finally noticing the futility of their cause, scamper away before they themselves get squished under tons of weight. Such devastation only took place in a few minutes, with one bloodied Uintatherium then departing, unable to pass on his legacy this season, but perhaps in the future.

Such future is grim for quite a few lifeforms though. The baenid turtles, as many other animals, like the Patriofelis, will soon become extinct in terms of geological time. Such extinctions will result from the increasing fragmentation of the rich habitat we witnessed along this tale, a consequence of the gradual and interrupted, but yet sure cooling that has had its foundations established since India’s collision with Asia. As temperatures continue to drop, the planet will get progressively drier, opposite to what occurred during the ice ages of the long-gone Carboniferous. Despite such doom reserved for certain organisms, as usually occurs, some will be able not only to adapt, but also flourish. The equids are one such example. Their cursorial habits will only prove more and more useful in environments that turn increasingly open and less filled with trees. And with that we progress deeper into the Cenozoic, as the dynamic Earth approaches more and more our time, bringing with it escalating familiarity.

***

1-Viviparus

2-Aletornis

3-Herbaceous angiosperms

4-Boavus

5-Cucurbitaceae

6-Ferns

7-Chimaeroblattina

8-Sinopa

9-Lauraceae

10-Vulpavus

11-Sabalites

12-Saniwa

13-Mesonyx

14-Orohippus

15-Tetheopsis

16-Diatryma

17-Uintatherium

18-Torosaurus

19-Patriofelis

20-Chisternon

21-Zygodactylus

22-Titanomyrma

23-Notharctus

24-Sapindaceae

25-Angiosperm forest (including Quercus, Cinnamomum, and Acer)

26-Sequoia