Start of an Eon

At about 507 million years ago, this is the middle of the Cambrian Period. What will eventually become the American state of Utah is now a shallow and warm sea. The continental configuration is unrecognizable. To the south pole, there is a landmass known as Baltica and, even further, the supercontinent Gondwana, which stretches into the Western and Eastern hemispheres, apart from also reaching areas above the Equator. Farther north (but still in fairly low latitudes) in the west, the continents Siberia and Laurentia can be encountered, with the latter sitting fairly close to the aforementioned sea. While the organisms that inhabit these waters may seem quite unfamiliar, many members of groups alive today can be easily found. Before observing the life in more detail, it is essential to elaborate on the Cambrian Explosion. This so-called “explosion” was not a physical one, but instead an event in which many animals, such as chordates and arthropods, appeared in expressive numbers.

While it was initially thought that this process was fast and somewhat immediate in terms of geologic time, it is now mostly thought it was more gradual, not an explosive, but a drawn-out and long process extending to before the Phanerozoic that actually was part of a larger early Paleozoic biodiversification process. Either way, it is undeniable a radiation of creatures took place during this time frame, which raises the simple question: Why? Perhaps the most significant trigger was the previously mentioned continental configuration, which fostered increased ocean oxygenation and large numbers of tropical, coastal seas, like the one on this trip, areas propitious for diversity. In addition, organisms themselves may have contributed to this process in another way: burrowers might have disturbed the formerly typical mats of microbes covering the seafloor, allowing for more mixing between the sediment and the water column, consequentially generating oxygenation of the substrate. While this likely was destructive for various metazoans adapted to the mat environments, this change probably proved important for the development of the more modern animals, adjusted to a looser substrate in full contact with the water above that would become increasingly more common along the following periods and eras.

After this brief explanation, it is finally time to explore the Cambrian reef that presents itself. Dispersed through the whole area are many Diagoniella, a genus of sponges. They get their nourishment using specific cells called choanocytes, which draw in water through the synchronized beating of flagella. Due to this, bacteria and sometimes even small eukaryotes are retained in extensions of the choanocytes' cellular membranes called microvilli, with the captured prey being then transported to amoeba-like cells known as archaeocytes, responsible for digestion and many other functions due to their totipotency. Also sponges, but with a very different appearance, are the Choia. These strange beings resemble spiky disks (almost 3 centimeters in diameter, with some attaining larger dimensions) attached to the seafloor by long stalks which come out from their underside. Despite these differences, the feeding method of these two members of the phylum Porifera is similar, as the Choia also rely on the passage of water to gain their nourishment, even though different sponge species can display also differing dietary preferences, some being generalists and others only consuming, for example, determined types of bacteria. 

Despite the correspondence in terms of appearance, the Chancelloria, possessing a mechanism of biomineralization (responsible for their numerous spike-like structures) very distinct from the one found in sponges, are not members of Porifera, probably belonging instead to a clade of basal animals, but not as basal as the phylum of the creatures they resemble, with which their dietary habits, based on filter feeding, also coincide.

Something moving inside the sediment can be noticed, being only denounced by the material it displaces and by two holes located near one another. Soon, however, part of it pokes out, a proboscis. The organ is extended: its owner, the stem priapulid worm Ottoia, likely mistook some chemical cue for prey. Just as fast as it emerged, it once again gets out of view, quickly retracting its hook-covered weapon and pulling the rest of its exposed body into one of the cited holes. Under the substrate, the 8-centimeter-long occasional predator forms a subhorizontal, U-shaped burrow and, from there, it can strike and capture something to eat if successful. In that regard, Ottoia are not picky eaters, consuming the living and the decaying, with carcasses being more than capable of aggregating a significant number of individuals. During these "reunions", the Ottoia that came looking for food may themselves get turned into it by their peers, with cannibalistic behavior also being seen in modern priapulids.

Not very far from this soft-bodied critter, but not in any form of danger, is an Asaphiscus, a genus of trilobite, and, a bit farther, some Elrathia, another trilobite genus, can also be seen. As a matter of fact, the Ottoia is in a riskier situation, since many trilobites do hunt worms, employing their spiny limbs to grapple and tear the soft victims. Luckily for the opportunistic invertebrate, this is not the case for the two genera of trilobite present, both belonging to the Ptychopariida group, containing mostly particle feeders, which consume detritus or graze on microorganisms present along the seafloor. The Elrathia in specific, although occurring in this area, are normally found in more extreme, low-oxygen environments bordering anoxic regions, where they ingest the sulfur-oxidizing bacteria that thrive in such gradient habitats. Trilobites as a whole are very recognizable arthropods, of which many fossils have been discovered. These animals survived through the whole Paleozoic Era (a considerable amount of time) and ultimately met their fate with its end. Right now, though, they are flourishing and can be found with ease in various locations across the planet. 

Focusing on a mat of the cyanobacteria Morania, other very intriguing lifeforms are observed. A few Wiwaxia, bottom dwellers up to 5 centimeters long, move on the carpet of prokaryotes, very likely feeding. While these peculiar creatures are not very gregarious, feasts such as these can gather a few individuals. They sport sclerites (the scale-like plates adorning their body) together with curious, protruding spines that serve defensive purposes and which the newborns of this species lack, only acquiring, later in growth, the structures, shed and replaced across the animals' lives. While originally uncertain in their classification, it is today thought that Wiwaxia are actually mollusks, even though it remains a mystery if they are basal members of Mollusca or if they are, in fact, early members of the more derived grouping of Aculifera (containing sclerites), sister to Conchifera (lacking sclerites), which includes gastropods and cephalopods for instance. 

Using their radula embedded with teeth, the Wiwaxia graze upon a true microbial society, with the cyanobacterial cells stacked together in various layers held by a slimy secretion composed of various materials the minute lifeforms excrete. These arrangements (a specific form of biofilm) create microenvironments with optimal conditions for the growth of their originators, providing increased protection and sometimes being associated with a division of labor between its participants (which may be of various different species, potentially resulting in quite distinct and contrasting layers). Some have even considered biofilms multicellular organisms in their own right, since many features typical of multicellularity are in fact truly present, even to the point of the microbial members of the mat jointly dispersing to colonize new areas. Either way, these associations are undertaken by a wide variety of microorganisms (in the modern world, 40 to 80% of bacteria and non-eukaryotic archaea may participate in biofilms, accompanied by several protozoans), with the extracellular matrix containing components ranging from carbohydrates, proteins, and lipids all the way to DNA. 

As most other bacteria, cyanobacteria display a cell wall that is organized in two layers, with the inner one being composed of a single one molecule: peptidoglycan, which is an association of amino acids and carbohydrate derivates that forms an intensely interconnected mesh around the bacterial cellular membrane, conferring protection against environmental stresses while also allowing for the passage of many substances due to its porous nature. The outer layer, separated from the peptidoglycan wall by a large fluid and protein-filled space, is curiously a second membrane, but it, unlike the cellular membrane, is asymmetric: while the inner side is composed of phospholipids, the outer one is constituted by complex molecules known as lipopolysaccharides, a combination of lipids and sugars. Negatively charged, these substances interact with positively charged calcium ions and form tight clusters which create an impermeable barrier to several potentially harmful substances that may be present in the extracellular space, with porosity being afforded by specific proteins, some of which function as channels. 

However, other bacteria, mostly those belonging to the phyla Actinobacteria and Bacillota, mentioned in future tales, do not possess such an external membrane and count only with the peptidoglycan wall, which is, for its part, much thicker though. Be the bacterial cell wall as it may, both configurations do not exclude the possibility of even more layers, which may come in the form of sugary (they can be capsules, which are important for holding water and preventing the entry of viruses, among other very important functions, and slime, which is less organized and less firmly attached to the cell, associated with some forms of motility) or proteinaceous coverings.

In regards to DNA, these cyanobacteria, such as other prokaryotes, have circular chromosomes (like most things in nature, there are exceptions, organelles of bacterial origin and some bacteria have been found with linear chromosomes). In stark contrast, eukaryotes have the linear chromosomes just alluded to. Having the DNA organized this way is crucial for the process of meiosis, a hallmark trait of Eukarya, and may help with better maintaining larger genomes, also characteristic of eukaryotes. Such extensive genetic material is the result of many non-coding portions (such as the introns, which may have contributed to the process of eukaryotic chromosomal linearisation as mentioned in the previous entry) and repetitive sequences. These repetitive sequences can be found, for instance, at the ends of a linear chromosome, in regions called telomeres, areas that are constantly degraded during replication due to chromosomal linearity itself and that, among other functions, represent a "fail-safe" to inhibit changes in more significant parts of the DNA. Even though there are mechanisms, like the enzyme telomerase, to circumvent this process of degradation (which varies greatly between eukaryotes and between cell types of multi-celled eukaryotes), it is paramount for regulating cell growth in some multicellular members of Eukarya and, consequently, perturbations affecting it can lead, for example, to the emergence of tumors, which usually show abnormal signs of telomerase activity.

Going back, Naraoia, blind arthropods related to the trilobites, are also moving through the environment. Their exoskeletons are non-mineralized (very unlike the exoskeletons in trilobites, hardened by calcium carbonate in the form of calcite), a feature shared with others in their clade: Nektaspida. They, some being a little less than 3 centimeters in length, are typical opportunists, scavenging and preying upon smaller, softer creatures, such as worms, shredded to smaller pieces by their sharp, ventral appendages before consumption. Consequently, while not under threat from the trilobites, the Ottoia, lest it suffer a grisly death, better not cross paths with these other creatures, relentlessly scanning their surroundings in search of the next meal.

Very noticeable also are Sphenoecium: housed inside the long tubes, live tiny colonial animals that subsist on the suspended organisms they filter from the water currents via arms lined with tentacles that are themselves covered in cilia, which, through their beating, help drive both water and food particles into the critters' mouths. Reproducing both sexually and asexually, they, despite their rather strange looks, are actually our closest relatives encountered until now. Being pterobrachian hemichordates, the Sphenoecium are members of the superphylum Deuterostomia, forming with echinoderms (examples include the very iconic sea stars) a sister group to chordates (in which we are included).

Suddenly, the largest lifeform encountered so far makes an appearance. It is a Peytoia, member of the radiodonts, a group of basal arthropods that includes the famous predator Anomalocaris. Like the latter, it is also predatory, reaching lengths of about 30 centimeters, but, unlike Anomalocaris, it specializes in eating bigger, less agile prey that roams throughout the seafloor, despite also occasionally attacking swimmers in the water column. The smaller inhabitants of this section of the sea have, despite this, no reason to fear, since it is just swimming by, slowly undulating up and down the flaps that adorn both sides of its body, not looking for any food: an unfortunate Elrathia was already consumed earlier this day. Similarly to its relatives, Peytoia has two frontal appendages, used to ensnare and manipulate food, an oral cone, which is the effective mouth, located on the underside of the head, and two compound eyes, potent tools to help find the next possible victim. Predators such as these are both a bane and a blessing for quarries like the scavenging ptychopariid trilobites, which feast on the excrement produced, in some cases, by the consumption of their former counterparts.

Particularly lucky trilobites may even survive a predatory encounter with a radiodont. However, they will likely take with them some lesions and while these may not seem like a big deal, especially when in contrast with the grim fate they escaped, the disruption of the most external layer of their integument leaves them vulnerable to colonization by pathogenic bacteria that are capable of degrading chitin, a structural carbohydrate found in various lifeforms, including in the cell walls of fungi and in the exoskeleton of arthropods, but absent in deuterostomes and in plants. While these microbes are usually just essential decomposers of the marine realm, being incredibly important for recycling the chitin that comes primarily from dead arthropods, they can become pathogenic in these specific situations. Although these infections are normally non-life threatening, if the bacteria gain access to deeper layers of the animal, they may disseminate systemically, which may prove fatal.

Finally, some of the last animals to be witnessed in this snapshot of the past are vertebrates, making them our closest relatives encountered so far, significantly closer than the Sphenoecium to be sure. These are primitive conodonts of the genus Hertzina and the two individuals being observed are spooked, frantically moving away from the Peytoia, which just came into sight. In general, conodonts resemble eels and are jawless, sporting characteristic teeth-like elements. Like trilobites, they also were present during the whole Paleozoic, managing to stretch all the way to the first period of the Mesozoic. However, unlike trilobites, they are currently not very diverse, but this will change in the following millions of years. 

Despite the great unrelatedness, these conodonts, as well as some of the seen arthropods, can be infected by a rather ancient family of viruses called the Polyomaviridae, being small, DNA-based, and unenveloped, their genetic material contained in a polygonal proteic capsid. The original polyomavirus was likely present in the last common ancestor between protostomes and deuterostomes (this common ancestor was a bilaterally symmetrical animal that, after diverging from cnidarians, eventually split into these two groups, with protostomes encompassing all non-deuterostome bilaterians): several species have developed since then, with a very gradual rate of change nevertheless, a rate that will remain low to the present day (though a few members of Polyomaviridae, not accounting for their divergence, will exchange genetic material, creating chimeric forms), a time in which these viruses still remain widespread, some of them implicated in cancer development.

Not far away from this reef section, but in slightly deeper waters, another curious scene has taken shape. Earlier today, a dead Peytoia gracefully fell to the sea floor, its various flaps dragging up as its body made its way down. With no sound, it hit the substrate and drew up a modest cloud of debris. Soon, various opportunists made their way towards the carcass. Now, it is incrusted with Naraoia, which completely cover and obscure the larger arthropod's remains, using their aforementioned sharp appendages to shred the meat of their former predator, voraciously consuming anything they can. Writhing in the marine soil are other scavengers: many, but many Ottoia. They are in such numbers some individuals even seem to make knots around each other, launching their proboscises not only at the decaying meat, but also at each other, larger ones consuming smaller ones whole, in an open example of their cannibalism previously stated. 

Attracted by the commotion, a live Peytoia joins the buffet. It comes cruising down and effortlessly snatches three glutton Naraoia, hauling them from the body of its gone peer and easily crunching their soft exoskeletons with its barbed appendages, then transferring the smaller pieces to its oral cone for them to be ingested. Unlike the Ottoia, it has no interest in eating another member of its own: though cannibalism is widely spread among the animal kingdom, it is quite rare as a whole, potentially because it greatly facilitates the transmission of pathogens. These are sometimes highly specific to their hosts and, as such, one organism has a higher chance of contracting an infection from a member of its own species than from a more distantly related one. This has been demonstrated even in ourselves, humans: the neurodegenerative disease kuru, caused by a misfolded prion protein (better mentioned in the previous entry), greatly affected tribes in Papua New Guinea that, in funeral practices, consumed body parts of the deceased, which were contaminated with misfolded prions.

With this, ends the first tale. From here, life will continue to evolve and change. The world will surely get more recognizable as time goes by, but there still is a long way to go until that point is reached. On a final note, it is a fact that a considerable percentage of everything which once inhabited Earth never got fossilized, which surely means that many things will be left undiscovered for good. Keep this in mind and ask yourself the following as we start this journey: What wonders has time perpetually destroyed? And so is a question unanswerable by its very nature, but yet still fundamental to consider.

***

1-Peytoia 

2-Asaphiscus 

3-Naraoia 

4-Wiwaxia 

5-Elrathia 

6-Sphenoecium 

7-Diagoniella 

8-Chancelloria 

9-Choia 

10-Hertzina 

11-Morania 

12-Ottoia 

13-Marpolia