A Brief Overview of Human Evolution

11 minute read

Within the deep-time context of Earth’s geologic history, or even life history, anatomically modern humans have been on the global scene for a relatively tiny amount of time — around 300,000 years. But how exactly did we get here?

About 7 million years ago, the lineages that would eventually produce humans and chimpanzees diverged. This point of divergence is referred to as the most recent common ancestor, a term that describes the individual from which two distinct species originate. This shared ancestor is the theoretical “mother” of both humans and chimpanzees, as the offspring of this individual mark the beginning of speciation down distinct phylogenetic, or evolutionary, pathways. In this case, one of these pathways happened to split once again, resulting in the emergence of chimpanzees and bonobos, our closest living non-human relatives. The other pathway gave birth to a long and circuitous evolutionary journey that would eventually produce us, Homo sapiens sapiens (aptly named for our wise-wiseness). This path was anything but deterministic, comprising a variety of pre-human hominins that very well could never have arisen, or produced humans whatsoever. In this article, we’ll synthesize some of the key developments in this evolutionary picture, providing some broad prehistorical strokes from which to better understand our origins.

A phylogeny of Homininae evolution over the past 10 million years.

According to most experts, the first undisputable hominin species is Australopithecus anamensis, whose fossils can be traced back to 4.2 million years ago (or 4.2 mya for short). Originating in Eastern Africa, A. anamensis was quickly succeeded by a series of other Australopithecus species, such as A. afarensis and A. africanus (famously known for the renowned “Lucy” fossil) who lived between 3.9 and 2.9 mya. There is strong fossil evidence to support the conclusion that Australopithecus species were full-time bipeds, including a number of human-like footprints from 3.7 million years ago preserved in volcanic ash in northern Tanzania. However, apart from their bipedality, these species bore little resemblance to Homo sapiens. Standing between 3 and 5 feet tall, Australopithecus species were likely fully covered in hair and had a brain only 1/3 the size of that of modern humans.

Things start to look slightly more familiar around 2.5 million years ago with the onset of the genus Homo. While H. habilis, the hypothesized descendant of Australopithecus, is the first widely recognized human species, our true interest lies even one evolutionary step further. Following H. habilis, H. erectus made their first appearance around 1.9 mya, bringing with them a number of key morphological changes. For one, H. erectus began approaching the stature of H. sapiens, standing between 146 and 185 cm tall, or about 5 1/2 feet on average. It’s also hypothesized that they exhibited significant hair loss, highlighting the growing importance of sweating as a form of releasing heat while hunting, foraging, or travelling on the hot savanna. Most importantly, their brain size was substantially larger than that of prior hominins, averaging about 1,000 cubic centimeters compared to Australopithecus Afarensis’s cranial capacity of 420-550 cubic centimeters.

These morphological adaptations brought with them a number of behavioral changes as well. Most significantly, H. erectus is the first human species that we know was capable of using fire. Additionally, H. erectus is likewise credited with the creation of the Acheulean tool industry. This toolkit is characterized by the hand axe, an implement of greater complexity that required more craftsmanship, and provided a sharper edge, than previous Oldowan tools (a simpler category of tools used by prior Hominins). These developments, along with the fact that many of H. erectus’s fossil sites are scattered with bones of medium- to large-sized game, imply some exciting trends about H. erectus physiology and psychology. Mainly, these bones and fire usage imply the possibility of: (1) a coordinated group hunting effort to take down large game, demonstrating the development of group sociality, and (2) cooked food.

H. erectus is the first human species that we know was capable of using fire.

Cooking food has two major effects. The first is that it makes food softer and easier to eat, thus encouraging a reduction in tooth size and jaw strength. This also frees humans from having to spend half our day chewing (as many of our contemporary ape relatives do). The second is that it increases the caloric content that humans can get from food, as less energy is squandered internally in the breaking down of raw foods into something digestible. This extra energy provides the necessary preconditions for brain growth, a highly costly organ energy-wise, which is evidenced by H. erectus’s increased brain capacity. With all this in mind, it may not come as a surprise to learn that H. erectus was highly successful, becoming the first hominin species to expand beyond Africa, extending into Asia as early as 2.1 mya and Europe by at least 500,000 years ago, if not sooner.

Over the following centuries, H. erectus eventually speciated into two other forms of Homo: H. heidelbergensis and H. neanderthalensis. These species generally continued with the predominant adaptive trends already set in place by prior hominin evolution, including larger brains, more complex tools, and increasingly gracile (slender) jaws and teeth. They also continued to exhibit increasingly complex social behavior, such as sexual division of labor, group hunting, and, eventually, even art and ornamentation. That being said, one substantial limiting factor to Neanderthals’ success was the fact that they possessed significant vocal limitations and were incapable of articulate speech, greatly limiting their ability for complex social interaction. However, although they are often depicted as grunting brutes, Neanderthals were in fact highly intelligent creatures who enjoyed larger brains on average than modern humans and showed many signs of complex culture. This is perhaps evidenced by the fact that there was significant interbreeding between modern humans and Neanderthal populations for thousands of years. As a result, today, the genome of all non-African human populations contain anywhere from 1-4% of Neanderthal DNA. Talk about a lasting impression!

A reconstruction of a Neanderthal man in modern attire. Are we so different?

Speaking of which, we now come to the part of the story that is perhaps of most interest to our (admittedly anthropocentric) mindset: ourselves. Fossil evidence suggests that anatomically modern humans first emerged about 300,000 years ago. After that, following a number of unsuccessful attempts at migration out of Africa, recent evidence suggests that humans first successfully expanded into Eurasia around 65-50,000 years ago in a single migration wave from which all modern non-African populations are descended (though other dispersal theories exist). H. sapiens then proceeded to spread throughout Eurasia and Oceania, displacing, outcompeting, or exterminating (pick your favorite theory) all other human species they encountered along the way, along with much of the megafauna (large animals). What’s more, H. sapiens had even spread into the Americas potentially as early as 40,000 to 30,000 years ago, thus occupying nearly all the habitable land on the planet. What was it that made these early Homo sapiens so successful?

As with many things, it’s impossible to distill this answer down to one reductionist cause. Humans have been so successful simply because of our, well, human-ness. That being said, although human nature continues to escape definition, it is thankfully not beyond description, and we can nonetheless identify a number of factors that have contributed to our ancestor’s success.

As we’ve been discussing, Homo sapiens had a number of important biological adaptations that began developing in prior hominin lineages. These include bipedality, opposable thumbs, and sweating, among others. However, two vital differences that set H. sapiens apart from prior human and hominin species are (1) a spectrum of physiological developments in the human vocal tract which allow for articulate speech and (2) large brains. Together, these adaptations produced highly intelligent creatures, who now could talk to one another… And the rest of the story practically writes itself. In an eyeblink, humans were suddenly enmeshed in a highly complex world of language, religion, politics, friendships, tradition – human culture itself was suddenly conceived. With culture and intellect on their side, H. sapiens were able to swiftly adapt to new environments, innovate better ways of hunting and gathering, and create and tell new stories to explain the strange world they found themselves in.

A cave painting in Argentina by H. sapiens. What were these early humans thinking when they made this artwork?

Early human lifestyle will be discussed in far more depth in a later article, but I’d like to end here by briefly commenting on one final aspect of the origin of human culture. Another crucial side effect of these changes was that an individual H. sapiens’s fitness was constrained not by biological factors, but by social factors. For humans, it suddenly became important not just how much food you got, or how many mates you had access to, or if a predator would kill you or not, but how much status you had in your tribe, if people thought you were a nice guy, or if Jessica liked you, or, like, like liked you…or maybe not. It became not just an affectation, but a necessity to keep track of the various complicated and ever-changing relationships in one’s social sphere, as humans exchanged not just physical, but social capital. In the end, the minds that made us so successful in our global conquest also came with a steep tradeoff, as hidden biases adapted to our evolutionary context continue to color our daily experience of reality to this day…but that’s a topic for another article.

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