By the time our planet was four billion years old, the rise of large plants and animals was just beginning. Complexity exploded around that time, as the combination of multicellularity, sexual reproduction, and other genetic advances brought about the Cambrian explosion. Many evolutionary changes occurred over the next 500 million years, with extinction events and selection pressures paving the way for new forms of life to arise and develop.

65 million years ago, a catastrophic asteroid strike wiped out not only the dinosaurs, but practically every animal weighing over 25 kg (excepting leatherback sea turtles and some crocodiles). This was Earth’s most recent great mass extinction, and it left a large number of niches unfilled in its wake. Mammals rose to prominence in the aftermath, with the first humans arising less than 1 million years ago. Here’s our story.

A planetoid colliding with Earth, analogous to (but larger and slower-moving) a potential impact between Swift-Tuttle and Earth. The asteroid that wiped out the dinosaurs had just 1/26th the energy that being hit by Comet Swift-Tuttle would carry, and that impact was enough to wipe out 75% of all species on Earth. (Credit: NASA/Don Davis)

65 million years ago, a massive asteroid somewhere between 5 and 10 kilometers in diameter struck our planet. It kicked up a layer of dust that settled all over the world, a layer that can be found today in our planet’s sedimentary rock. On the older side of that layer, fossils such as dinosaurs, pterosaurs, ichthyosaurs and plesiosaurs are abundant. Giant reptiles, ammonites, and large classes of plants and animals all existed prior to that event, along with small, flying birds and the tiny, land-dwelling mammals.

After that event, the mammals survived. With no larger predators to stop them, they grew, diversified, and experienced a population explosion. Primates, rodents, lagomorphs, and other forms of mammals, including placental mammals, marsupials, and even the egg-laying mammals are all abundant at the start of the Cenezoic epoch.

A duck-billed platypus (Ornithorhynchus anatinus) swims in the enclosure at the Melbourne Zoo in Australia. Despite the fact that it lays eggs, has a bill, and has a myriad of features we don’t typically associate with mammals, the duck-billed platypus is more closely related to us than it is to any extant bird or reptile. (Credit: Small/flickr)

Almost immediately, the primates began diversifying even further. 63 million years ago — just 2 million years after the demise of the dinosaurs — they split into two groups.

1. The dry-nosed primates, known formally as the haplorrhines, which developed into modern monkeys and apes.
2. The wet-nosed primates, known as the strepsirrhines, which developed into modern lemurs and aye-ayes.

58 million years ago, another big change occurred: the haplorrhines experienced an interesting genetic split, as the first novel and unique evolutionary branch became distinct from the rest of the dry-nosed primates: the tarsier. With its enormous eyes, it was uniquely well-adapted to see at night.

With its enormous eyes but a dry nose, the tarsier holds the distinction as the first haplorrhine to diverge from the mammalian lineage that would give rise to monkeys, apes, and eventually modern humans. Note that they have dry, rather than wet, noses. (Credit: Via Tsuji/flickr)

The niche it now occupied was sufficiently different from the remaining groups of our ancestors that they evolved differently from the rest of their cousins from this point onwards. This type of evolutionary splitting occurs every so often, and isn’t unique to primates.

Although we normally don’t think very much about our distant cousins and how they develop once they’ve split off from us, it isn’t just haplorrhines like us (and our direct ancestors) that underwent interesting phases of evolution. All throughout the past 65 million years — just as it was before that time — the various mammals, birds, plants, and other living organisms evolved together. Evolution is driven by environmental changes, and that includes all the floral and faunal changes that occur on our planet.

A reconstructed illustration of the protocetid Georgiacetus vogtlensis, a primitive whale-like creature dating from the Eocene period. (Credit: Nobu Tamura/ CC-by-SA-3.0)

55 million years ago, a sudden rise in greenhouse gases caused the global average temperature to swiftly rise, wiping out many deep-ocean animals and plants. This transformation left many large, unfilled niches in the ocean, paving the way for cetaceans (the large oceanic mammals) to develop.

50 million years ago, some of the even-toed mammals began evolving into sea-dwelling creatures. The artiodactyls may have all evolved from a single, common ancestor, or may have evolved independently. Animals like Indohyus, which dates to 48 million years ago, may have given rise to protocetids: shallow-water mammals that returned to land to give birth.

The early fossilized remains of Darwinius Masilae, known as “Ida,” was originally thought by many to be a missing link in human ancestry, but this specimen is almost certainly a strepsirrhene, not a haplorrhine like monkeys, apes, and humans are. Although it dates back to 47 million years ago, it is probably more closely related to a lemur than it is to a human. (Credit: Nachosan/Wikimedia Commons)

Right around that time, 47 million years ago, the primate Darwinius masillae existed, as the fossil Ida, preserved from that time, provides a spectacular example. Although this was originally touted as a proverbial “missing link” in human evolution, Ida is not a haplorrhine like us, but a strepsirrhene: a wet-nosed primate.

But another 7 million years later — 40 million years ago — an important development occurred among the dry-nosed primates: the New World monkeys branched off. Humans and our ape ancestors are descended from Old World monkeys; New World monkeys are the first simians (or higher primates) to evolutionary diverge from our lineage. They would go on to colonize most of South America, where they are still found in abundance today.

The golden-headed lion tamarin is an example of a New World monkey. This endangered species of animal is part of a lineage that split off from the Old World monkeys that humans are a part of some 40 million years ago. (Credit: _paVan_/Wikimedia Commons)

The Old World monkeys continue to thrive and successfully occupy their niches, while diversifying in body size and physical features. 25 million years ago, the first apes evolved, splitting off from the remaining Old World monkeys at that time. The apes — defined by the complete lack of a tail of any type — would go on to give rise to many of the close relatives of humans that survive today: both the lesser apes and the great apes.

The earliest ape to split off from the Old World monkeys was the Gibbon, a lesser ape that first emerged 18 million years ago.

Sometime between 14 and 16 million years ago, the first great apes appeared, with Orang-utans branching off 14 million years ago. The Orang-utans spread into southern Asia after this, while the other great apes remained in Africa. The largest primate ever, Gigantopithecus, first arose some 9 million years ago, only becoming extinct a few hundred thousand years ago.

The Orang-utans are some of the earliest great apes to split off from our hominid ancestors, which they did some 16 million years ago. Although they are true great apes like we are, having no tails, they are less closely related to us than gorillas, bonobos, chimpanzees, or any of the later-emerging members of the family hominidae. (Credit: ReubenST/Wikimedia Commons)

7 million years ago, gorillas branched off from the other great apes; they remain the largest of all the surviving primates.

The great apes split off in two directions 6 million years ago, with one direction giving rise to humanity’s ancestors and the other branch giving rise to chimpanzees and bonobos. The chimpanzee/bonobo branch remains unified for another 4 million years, with our closest surviving relatives — the chimpanzees and bonobos — diverging from one another a mere 2 million years ago.

Bonobos, along with chimpanzees, are the two species most closely related to human beings that remain on Earth today. Bonobos are incredibly social, but still are not truly bipedal, as they maneuver on four limbs frequently. An evolutionary split that occurred approximately 5.6 million years ago marks the divergence of these creatures from modern humans. (Credit: Eric Kilby/flickr)

But along the track of our direct ancestors, the developments were rapid and profound. 5.6 million years ago, the first truly bipedal ape, Ardipithecus, arose. Although it’s a controversial claim, the hand bones in Ardipithecus show evidence of it being a transitional fossil between the earlier great apes and the later Australopithecines.

Approximately 4 million years ago, the first Australopithecus evolved: the first members of the Hominina subtribe (a taxonomic classification more specific than family but less specific than genus). Shortly thereafter, the first evidence of stone tool use appears: presently at 3.4-to-3.7 million years ago.

A reproduction of Australopithecus africanus, based on the STS5 skull (“Mrs. Ples”) found in Sterkfontein, South Africa, dating to 2.7 million years ago. Australopithecus was the dominant hominid throughout Africa for nearly 2 million years, until the rise of Homo habilis. (Credit: Nachosan/Wikimedia Commons)

A critical evolutionary step happened a little more than 2 million years ago, as our hominid ancestors faced food shortages. One evolutionarily successful approach was to develop stronger jaws, which gave us the capability to eat foods (like nuts) that were otherwise inaccessible. But another approach was also successful: to develop weaker jaws and larger brains, enabling us to access the food.

While both groups survived for a time, the larger-brained group was more adaptable to changes, and they continued to survive. This is the evolutionary path that we think led to the development of the genus Homo, which first arose about 2.5 million years ago. Homo habilis, known colloquially as “handy man,” had larger brains than their Australopithecus counterparts and displayed far more widespread tool use.

The evolutionary family tree of modern humans, some of our more distant cousins, and many of the intermediary species that existed at one time, but that have died out and/or evolved into other life forms by the present time. (Credit: Dave Huth/flickr)

About 1.9 million years ago, Homo erectus evolved. This human ancestor not only walked fully upright, but had much larger brains than Homo habilis: nearly twice as large, on average. Homo erectus became the first direct human ancestor to leave Africa, and the first to display evidence of using fire. Homo habilis was likely driven to extinction more than a million years ago, as was the last Australopithecus.

Across the world, new examples of the genus Homo emerged, including Homo antecessor in Europe (which may be an evolved habilis or erectus, or an early form of heidelbergensis) about 1.2 million years ago, followed by Homo heidelbergensis some 600,000 years ago. Approximately 700,000 years ago, the earliest evidence for cooking appears; about 500,000 years ago, the first evidence for clothing appears.

The oldest fossil remnants of Homo sapiens now date back to 300,000-315,000 years ago, and were found in Morocco. This find, dating back to only 2017, pushes back our species’ origin earlier than the development of the Neanderthals, and suggest that we didn’t evolve only in East Africa, as previously believed. (Credit: Natural History Museum London/Nature)

Approximately 300,000 years ago, the first Homo sapiens — anatomically modern humans — arose alongside our other hominid relatives. It is unknown whether we descended directly from Homo erectus, heidelbergensis, or antecessor, although Neanderthals, which came slightly later at 240,000 years ago, most certainly came from Homo heidelbergensis. Modern speech is thought to have arisen almost as soon as Homo sapiens did.

It took 13.8 billion years of cosmic history for the first human beings to arrive, and we did so relatively recently: just 300,000 years ago. 99.998% of the time that passed since the Big Bang had no human beings at all; our entire species has only existed for the most recent 0.002% of the Universe. Yet, in that short time, we’ve managed to figure out the entire cosmic story that led to our existence. Fortunately, the story won’t end with us, as it’s still being written.

Evolutionarily speaking, human beings – or homo sapiens – have been around for a cosmic blink-of-an-eye: under half a million years. Based on how evolution works, it is unlikely there will be any humans left even just a few million years from now. (Credit: asdfgf/Wikimedia Commons)

Ethan is on vacation. Please enjoy this older article from the Starts With A Bang archives!