Omnis cellula e cellula. (Every cell comes from a previous cell)
— Rudolf Virchow, 1855
As the HBO series Westworld phrased it so succinctly: “Evolution forged the entirety of sentient life on this planet using only one tool… the Mistake.” A simple error in the code, one failure in one transcription, otherwise known as a mutation, is the first turn of a Rubick’s Cube. These simple errors in copying — given a template vast enough and a time table long enough, passed on through countless generations of genetic code — can ultimately result in all the diversity of life we see around us today.
“Over the course of four billion years, life has unfolded from minute specks indistinguishable from contemporary bacteria to great and integrated creatures, mushrooms and pterodactyls, whales and oak trees.” — Franklin M. Harold, In Search of Cell History, p. 223
All mammals on the earth today can trace their lineage back to one little reptile living some 200 million years ago. Before that all reptiles derived from one slightly divergent line of amphibian, and before that all amphibians derived from one slightly divergent line of fish. And in this manner every class of life merges with another further back on the family tree.
The chordate lineage in a nutshell: Flatworm > Fish > Amphibian > Reptile > Mammal.
How closely related are we to the other branches on our family tree? Well, the evidence is in the DNA. On the whole, you share roughly…
- 99.95% of your DNA with your parents (you are effectively a clone)
- 99.9% of your DNA with any other person on earth
- 96-98% of your DNA with chimps
- 90% with cats
- 82-84% with dogs (sorry, dog lovers; cats win by a whisker)
- 80% with cows
As a general rule, you may imagine any mammal to be 75% or more genetically identical to us, reptiles are under the 75% threshold, then amphibians in the 60s%, then fish, insects and invertebrates all lumping together by the low 50s. Interesting aside: we humans are about 50% genetically indistinguishable from a banana. Yes, you are half banana. Or a banana is half you… I’m not sure which is more disturbing. But this is why our bodies will more easily digest a hamburger than a salad… frankly, the hamburger is more closely related to us than the salad. Thus it is easier for our bodies to turn the cow into human than it is for our bodies to turn a plant into human, as it is further removed from us genetically.
Parallels Between Language and DNA, and the “Mutt Lineage” of Humans
Evolution… descent with modification. Every child looks like their parent, but not entirely; the detail is in that 50% of the .05% of difference! Factor this tiny width of a difference by thousands of generations and billions of individuals and the variations stack up. But evolution isn’t just about slight modifications in the genome. It also benefits from — and even requires — isolation. As we’ve learned in the past several decades, the tagline for evolution is less “survival of the fittest” as it is… “location, location, location.” Evolution and the emergence of new species (or “speciation”) is much more often determined by simple genetic drift in isolation. Put simply, things left alone will go their own way.
The evolution of DNA may be compared to the way in which human language tends to evolve. Words appear or disappear in isolated communities, meanings change, colloquial terms and different slang might arise. The Romance languages that dominate Europe today — Spanish, French, Italian, Portuguese and Romanian — all descended from one language… common Latin 1500 years ago. All five of these languages emerged out of one in nothing more than the slow-cooker of time and geography. In this way language families are not unlike similar genetic haplogroups: cut off any small population from another small population and their DNA will gradually drift away from the other population, like any once-shared language will drift apart, until eventually — much as a different accents evolve into different dialects and finally into different languages — the two populations will head their way into entirely separate species if isolated long enough. Human and chimp lines diverged some 6 million years ago, probably due to the geographic boundary presented by the Great Rift Valley. The more recent split between the chimp line and the bonobo line 2 million years ago probably came about by a similar geographic split. If one divergent group is not “brought back into the fold” (or cannot be, given geographic distances) that group simply continues to accrue genetic differences from any other, much like a fork in the road leads to paths that continue to diverge.
Our human species, too, seems to have experienced a split, as Homo heidelbergensis spread out of Africa some 800,000 years ago. Those heidelbergensis populations in Europe gradually became what we classify as Neanderthals, those in Central Asia became Denisovans, while those remaining in Africa became… well, us. Given enough time and geographic isolation, one human species just gradually became three.
Our species, Homo sapiens — anatomically modern humans — emerged in Africa about 200,000 years ago, and we began trickling out of Africa some 65,000 years ago. The story of our species’ journey across the globe has taken place in a far shorter timetable (less than a tenth of that heidelbergensis timeline), not enough to produce different species, but as anyone may attest, certainly different regional and geographic features. Some sixty-five thousand years ago ALL Homo sapiens were dark-skinned Africans. And in case you’re wondering, we do know exactly which African population birthed the rest of the world… as geneticist Spencer Wells and Genographic Project has demonstrated, all populations on this planet not living in Africa today can trace their genetic markers to the distant ancestors of the Khoisan people (the San Bushmen and the Khoikhoi people). The buck stopped here. This was the face of humanity some six hundred centuries ago, and these were the features that evolution would shape with great elasticity across the vast corridors of the earth.
Okay, so we left Africa, but perhaps the bigger surprise is that it took us so long to do it. Essentially, three quarters’ of our history as a species was spent in Africa. In the words of paleoanthropologist Chris Stringer: “African populations have the greatest [genetic] diversity, and people outside of Africa are essentially a subset of that variation.” (Lone Survivors, p. 182) All populations living outside of Africa show a much lower genetic diversity, suggesting one or more “bottleneck effects,” as populations left Africa in very small bands.
A friend recently asked me why the rest of the world would show lower diversity, and I guess it might seem counter-intuitive to what we see reflected in our vastly different faces of today… but the reason Africa features far more genetic diversity is because the entire population of the world today living outside of Africa came from very small emigrations of isolated bloodlines. Some 99% of Africa remained in Africa, the rest of the world today is made up of the 1% that left it. It’s like pouring out a few jellybeans from a jellybean jar; the jellybeans that you’ve poured out of the jar are but a small sampling and would not include all the colors still residing in the jar. Today a European and an Asian person may reproduce, but the resulting child still wouldn’t possess the diversity that an African child would from parents of neighboring villages. In the case of those of us who left Africa, the sands of the DNA hourglass invariably narrow at that 65,000 year-old bottleneck, while the African continent continued for the past 65,000 years a more free exchange of the older genes going back 200,000 years or more.
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.
The Milestones that Made Us Human…
- 8 million years ago — Genus gorilla (the ancestors of today’s gorillas) diverges from the line that would later produce chimps and humans.
- 6 million years ago — The genetic lines that will comprise chimpanzees (genus Pan) and humans (genus Australopithecus) diverge from one another.
- 3.2 million years ago — Today’s most famous Australopithecus specimen, Lucy, lives and dies without fanfare in East Africa.
- By 3 million years ago — Studies examining the divergence between gorilla lice and human pubic lice suggest that the Australopithecines have lost most of their body hair by this point in time.
- 2.8 million years ago — With a larger brain and a clear ability to fashion tools, genus Homo emerges out of the Australopithecus line.
- Between 2.8 – 1.8 million years ago — A growing variety of Homo species, living in isolation much like communities of gorillas today, gradually absorb or simply replace Australopithecines throughout the African continent.
- 2 million years ago — Homo erectus emerges, beginning a reign of two million years… representing the longest-lived and most successful human species (to date!).
- 1.8 million years ago — Erectus begins migrating out of Africa, marking what is probably the first human exodus out of Africa.
- Between 900,000 and 700,000 years ago — Homo heidelbergensis emerges, probably largely out of the African erectus/ergaster line. Heidelbergensis populations spill out of Africa in another wave of human expansion. Over the next several hundred thousand years these populations outside of Africa diverge into Neanderthals and Denisovans.
- 200,000 years ago — Anatomically modern humans — Homo sapiens (yes, our species) — emerge in East Africa, probably largely out of the heidelbergensis populations in Africa. If this seems recent, it is… Homo sapiens arise out of the most recent 7.1% of the history of the Homo lineage. So, 92.9% of human history predates our species, and for that matter, writing (recorded history) has only existed for 0.178% of human history. Oh, somebody drag me away from this calculator….
- Between 100,000 – 90,000 years ago — There is spotty evidence to suggest Homo sapiens are trickling out of Africa in early migrations.
- Between 70,000 – 65,000 years ago — The ancestors of today’s world-wide genetically traceable population leaves Africa, but this total exodus would probably not account for more than 1% of the total Homo sapien population at the time, the bulk of our species (some 99%) remains in Africa, absorbing archaic human populations.
- 40,000 years ago — There are no fewer than 5 human species living outside of Africa at this point in time: Homo erectus populations still surviving in eastern Asia, Denisovans in the central Asia, Neanderthals in Europe and Homo sapiens. Genetic evidence suggests interbreeding occurs sporadically (if not regularly) between Homo sapiens and the other three species, effectively and eventually absorbing them. The fifth human species, Homo floresiensis in Indonesia, is a confusing outlier that we in the 21st century don’t yet understand (Is this a Homo erectus variant? Does it represent a heretofore unknown Australopithicine emigration to Asia? Did Homo sapiens ever encounter it? We simply don’t know).