Our Ancestors

By ooorg

Created 17 Aug 2021  (Edited 17 Aug 2021 )


Just like all life, humans evolved from earlier species. One significant point in the evolution of Humans is the point at which our ancestors separated from the ancestors of our closest living relatives, the Chimpanzee and the Bonobo (which make up the genus Pan).

That ancestor of ours, Chimpanzees and Bonobos we call the Last Common Ancestor (LCA).1 They lived around 6 million years ago. Although LCA is an individual (or individual pair, the mother and father), we can think of it as a species too.

There have been quite a few species that descended from the LCA, in our descending line (and not that of Pan), but the only living one today is us, Homo sapien (or AMH, Anatomically Modern Humans). All of those historical species up to ourselves are categorised as Hominins. The Hominins, together with the Great Apes (and all their ancestors going back to the separation of the Orangutan ancestor, around 14 million years ago) are categorised as Hominid. They are also the Gorilla, Orangutan and Gibbon.

The (CH)LCA is an arbitrary taxon in the evolution of Humans apart from the fact that it is common to the living Pan genus and that we can know and study both the Chimpanzees and Bonobos, and the comparisons between us might help us to understand more of our ancestry. But the fossil record of Pan's ancestors is even more scarce than of our own. The dates of LCA are given here (fig 1.) as 6 million years ago (MYA), though they are posited as between 13 and 4 MYA. However, the speciation between the two species is thought to have taken millions of years and therefore the later dates are more likely to have finally separated into the two lineages. Examples of early fossils that might be ancestors of Pan, part of the ongoing speciation or even (but less likely) on the Hominin side are Sahelanthropus tchadensis (7MYA), Orrorin tugenensis (6.1 - 5.7 MYA) or even Ardipithecus ramidus (4.4 MYA). It is only once we get to the Australopithecines, that we can safely place them as our ancestors and not that of Pan.

There are obvious morphological differences between Modern Pan and ourselves, such as our comparative hairlessness, and the fact that Chimps and Bonobos are not fully bipedal as we are (they can walk on two legs but do not strictly do so). On such morphological comparisons alone, we can make assumptions such as that those in the fossil record that are more bipedal or less hairy than chimpanzees are closer in relation to us, and therefore no direct relation to Pan at all.2 But by knowing very little of the LCA, like just how hairy or bipedal they were, we cannot know for sure, by morphological comparisons alone, whether or not an ancestor (or morphological hominin from the fossil record) is part of the Pan or Hominin lineage.3

We don't know, therefore, just how much the Modern Pan has changed since the LCA and cannot assume that is has changed as much as we have in the same amount of time. The fallacy that 'we evolved from Chimps', assumes that Chimps have not evolved at all (at least since the LCA), and it is we that are evolving. While this has nothing to do with science today, it might have been in the past and still contribute mythologically to perceptions of Human exceptionalism. While Humans have been exceptionally successful, it is not through design and evolution is also certainly not unique to humans.

There is a scarce fossil record of Hominins, so any new discovery alters the picture we have, but to date we think of the early Hominin phase containing the genus Australopithecus, which lived in Eastern and Southern Africa between around 4-2 million years ago. The Australopithecines, generally split into two lines - the gracile and the robust. Homo Sapiens (AMH) descended from the the more lightweight gracile australopithecines like Australopithecus afarensis, the famous fossil example known as 'Lucy'.

The Australopithecines are the first Hominin to be significantly bipedal. We know fairly little about these upright ancestors of ours. But we have fossil records so we can know their cranium size and their physiognomy to a certain extent. The 'robust' versions of the Australopithecines became the genus 'Paranthropus', with examples such as P. robustus and P. boisei. While we did not ourselves descend directly from these other species, they are part of the Hominin family and unfortunately died out. The story of how we came to be the only living Hominin is one to perplex us, but it wasn't always the case for the early Humans (other species in the Homo genus) that they lived alone.

After the Australopithecines, came the genus Homo, and the archaic humans - several species which lived alongside each other. Homo habilis and H. rudolfensis were early examples. Then came H. ergaster, the early (and specifically African) version of the most successful and long-lived H. erectus, which left Africa and expanded through Europe to Asia. Ergaster and Erectus are very similar and considered a single 'chrono-species', however Erectus eventually did evolve a slightly larger brain, albeit over a nearly 2 million year period, such was the success of the species. In fact it is thought to have still lived (in places such as Java) around 115,000 years ago. It would have lived contemporaneously to other Human (Homo) species that it would have shared an ancestor with, such as H. heidelbergensis (which evolved from Ergaster back in Africa and is a more direct ancestor of ours), H. neanderthalensis (Neanderthals) and Denisovans - who both also evolved from Heidelbergensis - and us. Erectus is a direct ancestor of AMH, though whether we ever came into contact with them directly can as yet only be imagined. Incidentally, another Human species evolved from H. erectus into Homo floresiensis, named after the island Flores, and lived until 50,000 years ago. H. floresiensis was a diminutive Human, at a little over 1m tall.

It is certain, though, that Neanderthals and Modern Humans lived alongside each other. They had both evolved from a common Ancestor, Heidelbergensis, who had themselves left Africa for Europe about 600,000 years ago before evolving into Neanderthals (and Denisovans in Asia). Then we evolved from a remaining Heidelberg population in Africa about 200,000 years ago. By the time those of us had left Africa ourselves to Europe and Asia, the Neanderthals were already there, in Europe, with the Denisovans in Asia. We can know for sure that we lived alongside these other Humans because many people around the world, outside of sub-Saharan Africa, have Neanderthal DNA (1-4%), as have Denisovan DNA (up to 5%), more commonly in Asia and Australasia.

However, after the last Homo erectus and Floresiensis, the Denisovans and the Neanderthals eventually died out. The last known Neanderthals died out in Southern Spain around 30,000 years ago. It is thought for a combination of reasons but most likely the fragmentation of populations, climate change and losing out to Modern Humans.

So, while we know almost nothing about the (CH)LCA, the ancestor we share with Chimpanzees and Bonobos, we know that our most recent ancestor was Homo heidelbergensis, who had also evolved into the Neanderthals and Denisovans, who mixed together and also mixed in part with most of us today. And although we inherited physiological traits like a bigger brain and full bipedalism, we also inherited many things that we think make us distinctly human – the cultural – like tool use, clothing and fire control. While not our own invention, these things would begin to have an evolution of their own during the time of the Modern Human.

1. Technically it is the CHLCA (Chimpanzee–human last common ancestor), but we can just call it LCA as it is the most recent common ancestor of humans with any living species (Chimpanzee / Bonobo).
2. If we take evolution to be a linear process: that is, if a species might lose its hair or become bipedal, it is assumed that it will continue to become fully bipedal or hairless, rather than 'change it's mind and go back to being hairy and quadrupedal. This is unsubstantiated.
3. Morphology was once the best way to deduce knowledge about fossils but it is unreliable and has been scientifically superseded by genetics. However, morphology is still useful in determining factors from the fossil record. But things that look similar are not necessarily 'related'.