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A Narrative of Human Evolution!From Abiogenesis to Anatomical Modernity"Introduction!

This treatise attempts to cover in brief the history of human evolution from abiogenesis, that is, the origin of life on earth 3.5 to 4.0 billion years ago, all the way to anatomical modernity, that is, the point at which anatomically modern humans emerge on earth 200 thousand years ago. That is a period of time spanning 3.999 billion and 800 thousand years, a preposterously long time. We are going to attempt to cover this almost 4 billion year history in a brief narrative."

It must be fundamentally understood that evolution is not a linear process. It is a branching process by its very nature. For example, humans and chimpanzees share a common ancestor about 7 million years ago. That means that the offspring of the common ancestor took multiple paths, but here in this narrative we follow only one of these paths towards the anatomically modern human and ignore the rest. By logical extension this means that all creatures living today derive from a Last Universal Common Ancestor (LUCA) which was a simple single cell organism that lacked an organized nucleus. This branching is how tiny single cell organisms at the dawn of time over the course of millions of years gave rise to the diversity of plants, animals, bacteria, archaea, etc that we see now."

When I say common ancestor, I do not mean that it was the first or only creature of its kind during its time. LUCA, for example, is highly unlikely to be the first life form, or the only. It just happens to be the last life form in all history from which all creatures living today descend."

Abiogenesis!The Earth before Life!

4.8 billion years ago I’m not a geologist, so the geological history of the earth isn’t my strong point. From what I

have gathered, the earth along with other planets formed from the collusion of dust particles in a great proto-planetary disk of rotating dust that surrounded the young sun. Apparently the organic molecules necessary for the development of life emerged in this disk."

The Hadean Eon, the first eon of the earth’s life, is appropriately named. It seems that the earth at this time was very hot, with a thin, molten crust as the planet still formed, and rife with volcanic activity. The oceans, when they eventually formed, were acidic, and the atmosphere lacked sufficient oxygen to support modern life. A sister planet slammed into the earth during this time, throwing up pieces of the earth into orbit to form the moon, which brought stability to the earth’s rotation."

4.0 billion years ago The earth was lifeless for about a billion years since its formation 4.6 billion years ago. It

would not have been possible for life to successfully form earlier than 4 billion years ago (4

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billion years being the beginning of the Archaean Eon) due to the high amount of hostile planetary activity, including the collusion of meteors into the earth’s surface, which would have been enough to sterilize the planet and undo all progress in life’s formation. It could very well have happened, but if the earth was sterilized by meteorite trauma, we could not possibly have descended from that iteration of life, and so I cannot count it as a part of this history."

The low-oxygen atmosphere of the lifeless earth was highly inappropriate for sustaining modern life, however, it seems that in order for life to form in the first place, a lack of oxygen was essential, because the presence of a high amount of molecular oxygen would have prevented the formation of organic molecules."

3.7 billion years ago The details are hotly debated, but it seems that simple organic molecules, or monomers (the

building blocks of complex organic molecules) eventually formed in a sort of primordial soup that concentrated at the shores and vents of the oceans of the lifeless rocky earth. These monomers joined together and formed chains of polymers, complex organic molecules, although it is unclear as to how this was possible. As a result, nucleotides emerged, and eventually gave rise to self-replicating nucleic acids."

From Molecules to Life!RNA was among the first of the self-replicating nucleic acids to form the basis of some form

of life (it may not have necessarily been the first), as it is capable of carrying small pieces of genetic information and building proteins out of them. Life on earth was thus an RNA world, and with my lack of knowledge in this particular subject, I make the guess that the life forms were simple cell-like collections of small RNA genomes surrounded by the proteins they developed, encased in lipid bilayer membranes."

With the rise of the genome came the rise of the process of evolution itself, which is defined as the change in the genetic traits in a population of organisms over time. RNA molecules came in competition with each other to remain stable and replicate. Genomes that are more successful at replicating rose and became numerous, while genomes that failed to replicate were superseded and eventually displaced. It is possible for genomes to change because the process of replication is not perfect, sometimes spontaneous changes happen, and this is called a mutation. Most mutations are useless, many are harmful, but sometimes a mutation can be very helpful for survival and/or reproduction of the population that carries the trait. The history of evolution is thus a history of favorable mutations appearing in populations and then rising to prominence because they aided in the organism’s ability to replicate."

RNA is not a stable enough nucleic acid to support long genomes, and so, once DNA arose as a more stable form of carrying a genome, the RNA world became displaced by the DNA world, and RNA took on exclusively the role of building proteins in the DNA-based life form. The rise of DNA made the development of more complex life forms (and hence, more complex ways of self-replicating successfully) possible, leading to the eventual development of the first prokaryotes, and among them, our Last Universal Common Ancestor, LUCA."

Early Prokaryotes!The Last Universal Common Ancestor!

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3.65 billion years ago The last universal common ancestor was an unorganized ring of DNA encased within a cell

wall, meaning that it was a prokaryote, and it would be indistinguishable from a modern bacterium if you were to place them side by side. It used RNA to build proteins, and ATP to transmit energy within the cell. The cell generated ATP through the movement of protons across a membrane, a process called chemiosmosis. Most importantly, the cell could reproduce by replicating its own DNA and then dividing into identical copies, a process called mitosis."

Archaea!As this little guy multiplied, it eventually gave rise to two clades: the Bacteria, and the

Neomura. Bacteria are self explanatory, they remain prokaryotes (small cells lacking an organized nucleus) forever, but very interesting things would happen to the other clade."

My guess for the Neomuran ancestor was that it resembled a generic Archaeon. Archaea appear superficially similar to bacteria, but they are fundamentally different. They are actually more similar to us than they are to bacteria, not only in genetics but also in several ways such as how they replicate and repair DNA! The Neomurans are notable for lacking the peptidoglycan in the cell walls that is present in bacteria."

The Rise of Eukaryotes!The Organized Nucleus!

The Neomurans split into two more clades, Archaea and Proto-Eukaryotes. The archaea remained lacking an organized nucleus, but something very interesting happened in the sister clade!!"

The proto-eukaryote grew in size, so that the membrane folded within itself and wrapped around the genome, forming an internal endomembrane system and an organized nucleus."

2.3 billion years ago Now, the environment over the time since life’s formation up til now was changing. Thanks to

the activity of so many life forms releasing oxygen waste into the atmosphere, the atmosphere of the earth came to be full of oxygen. All the lifeforms that depended upon a low-oxygen environment to survive were now in danger! This included the proto-eukaryotes, which were now at risk of dying out also thanks to this first “pollution” crisis."

The Mitochondria!

1.85 billion years ago Fortunately for us, otherwise we would not exist, another very interesting thing happened to

the proto-eukaryotes. An oxygen-breathing bacterium found itself inside a proto-eukaryote for one reason or another. The really interesting thing is this: instead of one destroying the other, they developed an endosymbiotic relationship! The eukaryote was able to use the bacterium’s oxygen-breathing ability and the bacterium was in turn protected and nourished by the

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eukaryote. As a result, the eukaryote was able to survive in the new environment, and eukaryotes that lacked the bacterium eventually went extinct."

This is how the mitochondrion, the organelle within our cells that is responsible for cell respiration, came into being, and the fact that it was originally a bacterium is the reason why it has its own DNA, and why that DNA resembles the DNA of bacteria."

Unikonts!To propel themselves through the aquatic environment, the eukaryotes developed little

whipping tails called flagella. Two varieties dominated: the unikont, which had one flagellum, and the bikont, which had two. The bikonts would go on to be plants and their relatives but thankfully we’re not bikonts so we don't have to worry about covering any of that."

The amoebazoa split off from the rest of the unikonts and came to lack flagella, leaving us with the clade ophistokonta. These guys had their flagella located at the rear of their bodies, and they would give rise to fungi and animals, that is, us, and the assorted organisms more related to fungi and to us."

The ophistokonts split off into two clades, the holomycota, which are the fungi and the organisms more closely related to them, and the holozoa, the animals and the organisms more closely related to us. If I were to make a guess based on my minimal knowledge of the nature of fungi, the ancestral holomycota may have had filamentous pseudopod structures of some sort, but it seems that the same was true of the holozoa so I really don’t know."

The holozoa split into two more clades, the filiozoa, organisms that retain the filamentous structures, and Mesomycetozoea, protists that lose them."

The Collar Cells!The filiozoa split again, The other group, Filasterea, presumably retains the original shape,

while in our group the filaments aggregate around the flagellum’s base to form a collar, giving the collar cell its name. This is really important because it means that we are really close to the origin of Animals!"

Rise of the Kingdom Animalia!

1 billion years ago The collar cell clade splits again, into choanoflagellates, and animals proper. It is interesting

to note that the choanoflagellate is structurally identical to the collar cells of sponges, primitive animals, which is why the collar cell is pretty important to understanding the origin of animals.