The majority of the genetic polymorphisms found in our species are found uniquely in Africans. Europeans, Asian and Native Americans carry only a small sample of the extraordinary diversity that can be found in any African village.
— Spencer Wells, The Journey of Man, p. 39
Given its larger human populations and its greater continuity of occupation, Africa has probably always had more genetic and morphological variation than other parts of the inhabited world, giving greater opportunities for biological and behavioral innovations to both develop and be conserved.
— Chris Stringer, Lone Survivors, p. 219
To revisit the parallels between the evolution of language and the evolution of DNA, the ancient “Click” languages spoken by the Khoisan people show a very high complexity, a complexity that mirrors the higher genetic diversity found in their DNA. “English, for example, has thirty-one distinguishable sounds used in everyday speech (two-thirds of the world’s languages have between twenty and forty), while the San !Xu language… has 141. While it is uncertain exactly which forces govern the acquisition of linguistic diversity, this figure is certainly suggestive of an ancient pedigree — in exactly the same way that genetic diversity accumulates to a greater extent over longer time periods.” (The Journey of Man, p. 56) Dr. Chris Stringer makes a similar point in his Lone Survivors. “Africa has the largest number and diversity of phonemes, and that number decreases as we move away from Africa.” (p. 218) The language essentially mirrors the DNA.
But importantly, as various populations of our species migrated out of Africa they seem to have encountered their long-geographically separated cousins. Neanderthals and Denisovans were still to be found in the Caucasus, Europe and Siberia, and as it turned out they had a role to play in our narrative. As the DNA shows us… where populations met, they exchanged genes.
Currently, three human species have had their DNA sequenced and the genome decoded; the Homo sapien genome was finalized in 2003, the Neanderthal genome was published in 2010 and the Denisovan in 2012. There may have been as many as 20 human species over the past two and half million years, but we have not yet retrieved DNA from other older species. What we have learned in the past half decade though is that today’s world population shows an eclectic mix of archaic admixtures, added to the genetic mix within the past 50,000 years. Anyone of European or Asian descent today averages 2-3% Neanderthal DNA, while Tibetan/Melanesian populations today may show Neanderthal DNA… and an additional 2-5% Denisovan DNA.
It is interesting to note that in the case of Neanderthal DNA, the roughly 3% in my genome is not necessarily the same as your 3%, so we actually have different puzzle pieces buried in our DNA. If we were to compile together the DNA of everyone alive today, it is estimated that we could be able to reassemble roughly 30% of the entire Neanderthal genome. Almost one-third of the Neanderthal genome is still active and being passed down through human populations today. The former blanket statement that Neanderthals went extinct must be revisited; it is about as accurate as saying the dinosaurs went extinct while ignoring the bird singing at your window.
Outnumbered 10 to one by modern humans, Neanderthals weren’t hunted to extinction by a supposedly superior species. They were bred out, genetically swamped.
— PBS, Nova: Decoding Neanderthals, 2013
Broad swaths of the world’s population today carry a legacy of Neanderthal and Denisovan DNA across the globe, to continents neither species ever saw or imagined, not unlike the lost luggage of a dead passenger traveling the globe from terminal to terminal long after his burial!
Even the African continent retained pockets of archaic humans until relatively recently. And wherever populations met, population-mixing seems to have occurred. In the words of Chris Stringer, “sex happens.” From his 2012 book Lone Survivors: “[Current] African populations also contain about 2 per cent of ancient genetic material, and this was input some 35,000 years ago, not from Neanderthals or Denisovans but from an unknown archaic population within Africa itself, which might have been separate from the modern human lineage for some 700,000 years.” (p. 250, emphasis added) Whether heidelbergensis, erectus, or something more obscure or exotic within the pockets of isolation in Africa thousands of years ago, this finding just underscores the point that between Europe, Asia and Africa, gene pools separated by hundreds of thousands of years found a way to reconnect, even if only to re-establish a toehold.
The truth is, though we are the last human species on earth today, there was never any one lineage that resulted in the “us” of today. The lines that lead to us were a porous, fluid and tangled tapestry; we are a mosaic of countless Homo and Australopith contributions.
Some of these evolutionary experiments died out, others came together and interbred. The ebb and flow of these genes through these groups was probably so complex that we may have to give up hope of discovering a simple linear evolution.
— PBS, Nova: Dawn of Humanity, 2015
There were probably no fewer than 20 Homo species in Africa over the past two million years, and it does seem likely that every one of them may have contributed at least a little bit to the bloodline we see today. There are more than seven billion humans in the world now, but over the course of our ancestral history Australopithecines and early Hominins existed in perilously low densities. We spent most of our history as an endangered species, not unlike gorilla populations today. Historically speaking, think of these early human species as living in small prides, very much like chimp and gorilla communities today; small isolated collectives with little interest in outsiders other than an occasional sexual encounter and exchange of genes. The smaller these isolated groups, the more any outsider DNA would impact the gene pool.
The descent of Homo sapiens is not unlike someone’s genealogy of today, where despite your single surname there was no single line that made YOU; if you go back far enough, genealogical lines criss-cross, separate and actually merge again. There are multiple rivers that flow into the gene-pool of you, and the further back you trace the streams, the more their courses branch, split, reconnect, narrow and broaden in unexpected ways. Much like an all-female line resulting in the extinction of a surname, the “extinction of a species” is an arbitrary designation, because the DNA simply carries on, contributing to an individual or species no matter what “name” they hold. In this case, “extinction” simply juggles the proportions of the genetic mix. Minute genetic contributions of archaic humans live on within our species today. Homo sapiens may be “mostly derived” from Homo heidelbergenis in the same way that someone might be “mostly Irish” in heritage… the reality is more complicated.
But geography and climate have played a role too, in reshaping the human form outside of Africa in the past 65,000 years, as pockets of Homo sapiens spread across the globe. If the gene pool is the hardware of DNA, then think of the genetic responses to geographic and climate change as the software patches. As populations of our species moved north their skin gradually lightened to filter in more necessary vitamin D, and conversely those who had been north for thousands of years and then headed south again (Polynesian populations, and Indigenous Americans — both North and South) began darkening once more. Different features arose in different regions and formed the physical and cultural distinctions we still see today.
Genetic variation is geographically structured, as expected from the partial isolation of human populations during much of their history.
The worldwide human species that we see today is merely a snapshot; but it’s a perfect test-case of 65,000 years of genetic drift and varying degrees of geographic isolation. Epicanthal eye folds, lactose-intolerance, skin tone and body stature; these are all just different evolutionary responses to different regional and cultural stimuli. Humanity has spread from one continent to six, and in the process the continents have reshaped humanity.
My mom asked me once if evolution is over and I replied no; evolution never stops. As long as reproduction through sexual means continues, “descent with modification” is an inevitable result. After all, if each generation represents a single random twist of the Rubick’s Cube it’s only a matter of time before any two cubes would cease to look alike. Consider for a moment blue eyes, a cosmetic detail well under a thousand generations old. Blue eyes are a very recent adaptation; related to a distinct mutation located in the OCA2 gene that first appeared between 6,000 to 10,000 years ago, and we can actually pinpoint this mutation to the geography of the Black Sea region. “Blue-eyed Humans Have a Single, Common Ancestor” was a headline in January, 2008. Blue eyes are an example of a genetic variation recent enough to be caught in the act, as it were, between being a regional actor and global one, having played a prominent role in the theater of Europe and Scandanavia, but largely not yet introduced into indigenous populations of Africa, Asia or the Americas. It might be odd to think of the spread of blue eyes in the same way one might think of the spread of a sexually transmitted disease, but it’s not an entirely inaccurate analogy. Mutations are spread by sex. The reality is that any two unrelated people of Western heritage today will likely share a common ancestor by 600 years ago, but a same person of Western heritage and a person of East Asian heritage might not share a common ancestor before 40,000 years ago. So mutations like the one in gene OCA2 have lacked the opportunity or timetable to enter that larger worldwide population before the advent of air travel.
Eye color, earlobes, toe lengths & wisdom teeth; these are all genetic features that are variable today. These are transitional mutations. The more variegated a species the more successful it is, and the more variegated the more variations it produces. Evolution is an accelerated cycle. Repetition leads to variation, and the larger the pool of repetition (i.e., the population) the more variations inevitably appear. In short, not only are we still evolving but that evolution is accelerating.