Are you curious about how life evolved on planet Earth? Well, grab a snack and a beverage and settle in for a few billion years. Here in part one, I am going to cover earth’s history from the start of life to the evolution of our hominin ancestors, from 4.6 billion years ago to around six million years ago. For some, this will be all new information and for others a bit of a refresher; for everyone, if you have questions, please don’t hesitate to ask.
The Hadean 4.6 – 4 bya
The Earth formed (or reformed after getting smashed by Theia) around 4.55 billion-with-a-B years ago. There were oceans and weather but no free oxygen and no life. The recently formed, rapidly receding, and still-molten moon is fifteen times closer to earth than today which may have made the length of a day mere hours depending on the strength of the tidal drag, the gravitational force of the moon on the planet’s oceans. In the shallow tidepools, in what Alexander Oparin called the primordial soup, the line between chemistry and biology blurs.
“But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts,—light, heat, electricity &c present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter wd be instantly devoured, or absorbed, which would not have been the case before living creatures were formed.—” —Charles Darwin in a letter to J.D. Hooker, 1871.
In 1953, at the University of Chicago, a student named Stanley Miller, inspired by Darwin’s and Oparin’s ideas regarding the chemical source for life worked with his professor, Harold Urey to recreate Earth’s early atmosphere in the lab. It included liquid water, which was heated and condensed to simulate the water cycle, with an atmosphere rich in methane, hydrogen, and ammonia. Finally, they included electric sparks for lightening. Soon the water turned dark as it filled with amino acids, the building blocks of RNA and DNA. The Urey-Miller experiment was the first of it’s kind to simulate the early atmosphere and so that amino acids could be produced by simple chemistry. Since then, more experiments testing the results of an asteroid impact, volcanic activity, hydrothermal vents with different atmospheric conditions show that biomonomers can be created from simple chemistry in many ways, and we have reason to believe it was happening all over our solar system as we have found sugars, lipids and amino acids on meteors.
The RNA World Theory posits the first life, something that is capable of replicating and evolving, was RNA. RNA, ribonucleic acid, is similar to DNA, deoxyribonucleic acid, in that it is a nucleic acid bonded to a sugar and can copy itself using amino acid base pairs. However, RNA is a single helix, and instead of thymine, RNA’s adenine bonds with uracil. To spare you a novella worth of reading on the subject, I offer you this fantastic video from Stately Clearly.
The Archean 4 – 2.5 bya
Our first fossils, from 3.8 billion years ago are in the form of stromatolites, which are mounds created by layers of mud from microbial films (which still exist to this day!). The current winner for oldest stromatolite goes to the Isua Supracrustal Belt in Southwest Greenland, the second prize goes to Dresser Formation of the Pilbara Craton in Australia which is 3.5 billion years old.
As an aside: It’s pretty easy to gloss over that 4.55 to 3.5 billion years and not realize what a huge amount of time that is. A billion years. It took one billion years for life to get established and one billion years ago there wasn’t even multicellular life on this planet– and consider that in that billion years we’ve had five mass extinction events. To put a billion years into perspective, consider the time one second takes to pass: one-Mississippi. One billion seconds takes almost 32 years to pass. The Earth is 4.6 billion years old, 4.6 billion seconds is the equivalent of 146 years. If the world was 146 years old, our first bipedal ancestor showed up about two and half months ago.
Okay, now that you’re sufficiently over-awed (or glazed over) let’s get back to life! Specifically, the winding roads of mutations that lead to homo sapiens. It’s also important to realize that our ancestors evolved in tandem with the plants, fungi, bacteria, and the earth, with its shifting continental plates, climate, volcanoes, and meteor strikes. I will be dropping in random factoids throughout to help place us in time and space.
The Proterozoic 2.5 bya- 541 mya
The first two groups or domains of life were Archaea and Bacteria, both examples of prokaryotes, or cells without organelles or nuclei, which split from a common ancestor 3.8 billion years ago. By 2 billion years ago another domain emerged, the eukaryotes, or cells with internal organelles. Eukaryotes began as giant prokaryotes that gobbled up smaller prokaryotes with beneficial skills, like creating energy from oxygen or sunlight (or both). This is an example of endosymbiosis and it is believed that eukaryotes organelles, like mitochondria and/ or chloroplasts, were originally free prokaryotes. This is why mitochondria within a cell have different DNA than that in the nucleus of a cell. If you feel yourself going cross-eyed while reading this, I recommend this adorable video by the Amoeba Sisters.
“We are shared, rented, occupied. At the interior of our cells, driving them, providing the oxidative energy that send us out for the improvement of each shining day, are the mitochondria, and in a strict sense they are not ours. They out to be little separate creatures, the colonial posterity of migrant prokaryocytes, probably primitive bacteria that swam into ancestral precursors of our eukaryotic cells and stayed there. Ever since, they have maintained themselves and their ways, replicating in their own fashion, privately, with their own DNA and RNA quite different from ours… without them, we would not move a muscle, drum a finger, think a thought.” –Lewis Thomas, “Lives of a Cell”
At 1.5 billion years ago the eukaryotes had split into three groups, plants, fungi, and animals— but you wouldn’t recognize any of them since they were still single-celled. By 1 billion years ago the moon had finally solidified but was still trying to make a hasty getaway. By 900 million years, multicellular life emerged due to communities of cooperative eukaryotic cells. Just as prokaryotes found living together beneficial, an example of mutualism, so to did communities of eukaryotes.
At 800 million years ago the first, simplest multicellular animal was placozoa which split into sponges and eumetazoa. Placozoa is believed to be the last common ancestor of all animals, however, there is some dispute about where it should fall in the phylogeny. The diagram I have created is based on the genetic phylogeny. However, other theories put placozoa between the sponges and eumetazoa, or on a different footing completely. It is a flat blob, somewhat concave, multicellular but very small (~3mm) animal that lived in the ocean or possibly in the mud of the ocean floor. Placozoa are a bit mysterious as their blobby nature has not left any fossils, at least none that have been discovered as of this writing, and they have never been observed in nature. The specimen pictured, trichoplex adhaerens, originally discovered in the 1880’s in a fish tank, and kept alive in labs ever since is where much of our knowledge of the animal comes from. The scientists keeping them alive aren’t even sure if they reproduce by replication or sexually. It has the simplest DNA and structure of any multicellular animal, being comprised of approximately two-thousand cells of four types.
Then around 770 million years ago, during a period cleverly named the Cryogenian, the Earth became a giant snowball, all life that survived, did so in or under the ice.
The Phanerozoic 541 mya- Present Day
After the thaw, a trend for spinal cords, or something approaching them, started 540 million years ago. Which is a huge deal, considering our last example of an animal was a blob of cells. Within 40 million years, animals began exploring the land, perhaps to watch the Appalachian Mountains forming on the ancient continent of Pangea. The land would have looked very bleak because there were no plants on land until around 465 million years ago, just in time for the O-S extinction event. Thanks, glaciers!
440 million years ago, bony fish split into two groups: ray-finned fish, the ancestors of most modern fish, and the lobe-finned fish, the ancestor of modern mammals, reptiles, birds, and marsupials. By 397 million years ago, some of the lobe-finned fish traded in their fins for feet and the tetrapods, four-legged animals, emerged. It was around this time that the oldest insect fossil of a Rhyniognatha hirsti was being created in the bit of land destined to become Scotland, which may or may not have had wings but it definitely looked a bit like a silverfish. And like a dress rehearsal for the big show, there was the Devonian extinction, which just dragged on in pulses for, like, 20 million years. Yawn. Then…
PERMIAN EXTINCTION. The big one. In fact, it’s the biggest one to date and it happened around 250 million years ago. Mass extinction means that 75% of the planet’s species are wiped out, which can take tens of thousands, or millions, of years. The Permian Extinction wiped out over 90% of species (land and sea combined). Fifty million years later, there was another mass extinction, because archosaurs. For more information on the mass extinction events, check out this article on World Atlas.
Around 140 million years ago, placental mammals, eutherians, split off from marsupials (gestational pouches were so last epoch). 10 million years later the first angiosperms or flowering plants appeared, which looked like magnolias. 105-85 million years ago eutherians divided into four groups, laurasiatheres which include ancestors of modern hooved animals, whales (which began as wolf-looking land-based creatures), bats, and dogs; xenarthrans which include the ancestors of modern anteaters and armadillos; afrotheres, ancestors of modern elephants and aardvarks, and euarchontoglires, which included the ancestors of modern primates (us!) and rodents (check out the infographic below).
The euarchontoglires broke up 75 million years ago when primates split from the ancestors of rodents and lagomorphs (rabbits, hares, and other fluffy-hoppy creatures). The rodents would like us to know that they have done just fine without us; they make up 40% of all mammal species, including the noble capybara (we all have our favorites). Around 70 million years ago, grass had evolved but not grasslands, meanwhile, India was racing towards Asia and Europe was still trying to get itself together.
And then… the K/T EXTINCTION. (I suggest reading that in your movie trailer voice.) At 65 million years ago the earth experienced another mass extinction event, called the K/T or cretaceous-tertiary extinction when a meteor named Chicxulub slammed into the earth and blotted out the sun. In terms of mass extinctions, it is probably the most popular. It didn’t kill all the dinosaurs, but it did knock that chip off their shoulders and let us mammals have a turn. But don’t get too smug next time you’re indulging in an order of apex-predator nuggets because we’re probably next.
It might seem like we’re getting really close to humans, but we’re still about 64.8 million years away from us. Besides, we still need to sort out noses. At 63 million years the primates divided between wet-noses, the strepsirrhines and the dry noses, haplorhines. Now onto the simians!
But first, the Paleocene/Eocene thermal maximum at 55 million years ago after the earth warmed to a global average of ~4-5°C hotter than today resulting in hypoxic (low oxygen) oceans killing marine species and fundamentally altering ocean currents. The funny thing about greenhouse gases is that they’ve killed before and they’ll kill again, only this time we’re helping. Go us! If we were to just look at the oceans this would be a mass extinction event but on land, it was more like a big party. Life was diversifying at record rates.
Now, the simians, noses continue to be a defining feature. Around 40 million years ago the New World Monkeys (Platyrrhines, which means flat noses) became the first simians, or higher primates, which our species eventually evolved from (clearly we had a stake in naming). At 25 million years ago apes split from Old World Monkeys (Catarrhines, or long noses). We’re in the home stretch! Then the gibbons split off (18 mya), then the orangutans (14 mya), then the gorillas (7 mya) and finally the hominins, our bipedal ancestors, split from chimpanzees and bonobos around 6 million years ago.
Stay tuned for part two, where I will cover the next six million years of human evolution. I think three billion years of history is quite enough, for now, let it soak in.
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