Today, we literally, walk alone (at least on two feet) and are the last of a long-lasting line of creatures known as hominins. Our lineage dates back 6-7 million years ago. Over that time we have gone through many changes as animals, and it is only in the last 100 years that we have really begun to study and learn about our evolutionary history. As we learn more about our past, our future becomes more clear, and we can learn ways to apply the lessons of the past to the problems of the present.
While today, there are only four species of Great Apes left walking the planet today, only of them are bipedal in their form of locomotion, or that they walk on two legs. That would be us, Homo sapiens. This, as well as many other aspects of our lives, bodies, and cultures, make us a special species, possibly the most unique to have ever walked the earth (at least the most successful). But we were not always alone. In fact, being the only bipedal hominin is a relatively new development in the course of Earth’s history since hominins split from our last common ancestors with Chimpanzees some 7 million years ago.
Today, we dominate the world, with over 7 billion of us, no other species of our scope or size have reached these proportions, to the point where we strain the ecological resources of the Earth almost to the tipping point. Something about the way we go about doing things has allowed us to survive past many challenges. Disaster after disaster that wiped out other species of hominoids, our close relatives and ancestors.
There have been many branches of the hominin family tree, or braided stream as we will discuss, and all of them ended in dead ends except for our line. Why is this? Why are we the only living hominins, and for long how has it been like this? Today we will be exploring some of the ideas behind this, in an attempt to understand the complex landscape of the last 7 million years of hominin evolution.
Not so long ago, we were not the only species on Earth, in fact, many other hominin species were competing and coexisting in Africa for millions of years. Some are more successful than others, but all-important to the human story. It was long thought that human evolution was a linear path, that we had one species, which evolved into another, and so on until you reach the pinnacle, which would be modern-day humans. This is expressed explicitly in the “Progression of Man” illustration. However, as we are discovering over the last few decades, this is not how it was. In fact, it was quite the opposite. As we are learning, it is more likely that there were multiple species of hominins walking on two legs throughout Africa. From Australopiths to Homo, many species coexisted and could have, and in fact, did have contact with each other over the millions of years. With each new discovery, our place as unique creatures in the animal kingdom decreases.
For one example, we have the footprints at the site of Laetoli, Tanzania, which was discovered by Mary Leakey and her team, which has some great evidence to support this view. There are millions of tracks at this site or footprints, and some of them are even hominin, without a doubt. But to whom do they belong? That is the question, one of the sets of tracks we are pretty sure belonged to A. afarensis the species that Lucy belongs to. However, another track is right near it, and laid down around the same time, are the prints of a completely different biped. These two bipedal animals could have looked at each other across these plains. It is an amazing thing to think about as described by Dr. Ellison McNutt in my interview with her, as well as her recently published a paper about the Site A prints.
The evidence is mounting that hominins created and lived in multiple niches at once, coming in and out of the braided stream that we are learning is a better definition than even the family tree analogy.
It is through these discoveries that the picture of our origins becomes more clear. As we see how we became bipedal to being with, we can see how other adaptations followed, which help make us who we are today. At some point along the line of our cousin, and ancestral evolution towards becoming human, our genus had to eventually set itself apart. Our genus, Homo is one that we believe directly follows the Australopiths, and it is with Homo Habilis which lived about 2.5 million years ago, that we see the first species in our genus. With a brain size larger than those of the australopiths, and more habituated bipedal anatomy, Louis Leakey and his colleagues argued that this hominin ought to be the first in our family line, and after a great deal of discussion and debate, it was finally accepted taxonomically.
While Homo Habilis has had its place challenged many times, it remains to be the first recognized member of our genus. But it would not be the last. Not only did H.habilis possibly lived with other ancient humans, such as H. erectus towards the end of their longevity, but they also coexisted with various species of Australopiths, such as sediba, and others. From the very start of our genus, we were not alone on the plains but accompanied by many various creatures that would have looked similar to our contemporaries at the time. It truly was a Planet of the Apes, and it was only going to get more complicated, as more gaps in the fossil record are filled, we seem to find even more spots that are missing.
One of the biggest misconceptions over the last hundred years or so about human evolution is that it was one definitive step at a time. You had this species, it led to this one and then died out. They did not coexist and were not contemporaries. It was more of a ladder up to the top, where European Victorians believed were stood. Since then, and with new technologies, the blurry image of human origins is becoming slightly more clear. We now know, beyond a doubt that multiple species of humans coexisted for almost all of hominin history, except the last 40ky or so. From fossil and dating evidence, we can trace much older fossils of our earliest progenitors to living around the same place, around the same time. Then of course there are examples of snapshots in time, which are so rare, at Laetoli where there is near indisputable evidence that there were two types of hominins walking in the same field, within hours, if not minutes of each other shortly after a volcanic eruption.
It was only when the Neanderthals went extinct, sometime around 40,000 years ago, that we were truly the only humans left on earth. (Save a few populations that could have been modern hybrids we are still learning about, such as the Red Dear Cave People) Even just 100,000 years ago; we had H. sapiens, H. florensiensis, aka the Hobbit, H. luzonensis, Neanderthals, and Denisovans. Possibly even a third branch of the related Neanderthal clade in Asia, represented by the Harbin crania. Some populations of H. erectus may even have survived in some parts of southeast Asia. Slowly these species would die out, leaving us as the only survivors, and while there are many reasons why we can not be sure as to what led to their extinction. All of these hominins have a great deal in common, in anatomy, capabilities, and the reasons for why they went extinct. Each of them, including ourselves, is a mosaic of evolution, meaning certain parts are “more evolved” or better adapted than other parts we may have inherited from our ancestors. Derived vs. Acquired traits. So, instead of one hominin leading to another, and then going extinct, and event instead of a family tree to represent all of our cousins and ancestors, a better analogy that has been used for the last few decades, is that of a Braided Stream.
The idea of a Braided Stream, which is growing ever more popular, shows that traits and aspects of human evolution came in and out of the fossil record and our timeline, for various reasons. As they were needed, as they were not, they were discarded. An example of mosaic evolution, we can look at H. naledi with very modern human-like aspects, this creature was still suited for climbing in the trees, thanks to their long curved fingers, but their upright posture and long arched feet show that they were great at terrestrial bipedalism as well (walking on the ground like us), these features combined are called mosaic evolution (along with many other possible combinations).
The braided stream of human evolution has given us a much better way of explaining where, and when other hominins come into our story, how they evolved, and what traits they had. But why then, if these other hominins were so similar to us, are we the last ones left? While there is no absolute answer, to researchers it seems to be pretty clear cut. We did not drive other hominins to extinction, such as the Neanderthals, as some would like to believe, but rather we just out-competed them. We are very good at finding, and creating ecological niches, anywhere in the world to a level never seen before, which allows us to not only survive but thrive almost anywhere in the world. Other humans could not do this, and as the climate changed, and as they had to move around to maintain their lifestyles, as a species or as a small group, misfortune would soon befall them, the natural misfortune of natural selection.
These groups simply could not compete with Modern Humans on a social, cultural, or skill level, and this led to either their extinction, leaving us, or their assimilation into our own people, such as with the Denisovans and Neanderthals. We may never know what put the final nail in the coffin for these seemingly very successful species, who had been on the Earth much longer than we have so far, to go extinct. As we get more clues, one day perhaps it will be clear.
Well, I hope that answers some questions about whether or not a straight progression for humans, or really any species works, as we can see it does not. A much more appropriate example of how hominin evolution occurred is the Braided Stream hypothesis. Regardless, a few things have become clear in the last few years that were once doubted highly, and that is that multiple human-like creatures lived on the Earth at one time, interacted, bred with each other, and created new types of humans. All through the world, but especially within Africa, before migrating out. With evidence like that from Laetoli, Tanzania, we can actually see a moment in time, as if transported there ourselves, and can see that these footprints were laid down around the same time and that they do not belong to the same species. To me, this is fascinating, and I hope leads to many more years of research on the topic to come!