As recently as ten years ago, the single-origin and multiregional theories of modern human evolution were neck and neck, with perhaps more supporters for the latter theory, which postulates that very old (approximately 2.5 million years ago) archaic humans spread throughout the world, and humans living in different regions evolved separately into their current state. The fact that there is so much similarity among humans of different regions may be due to interbreeding throughout the history.
It was not until the rapid democratization of genomic analysis tools that the evidence firmly settles the argument in favor of the recent single-origin theory. This theory says that a single tribe/family was the only ancestors of all modern humans living today. Small groups of the tribe left East Africa and spread all over the world with astonishing speed and efficiency to become Homo sapiens. The exodus occurred as recently as 60,000 years ago, probably after a climate-related bottleneck (eg, the Ice Age) that cut their size down to a small number of blood relatives.
As these early humans migrate to northern Africa, the Middle East, Central Asia, Europe, northern Asia, and the Americas; and to Indian Ocean, southern India, and then the South Pacific, they replaced many other human species who had left Africa much earlier and inhabited these regions. It sounds incredible, doesn't it? The earlier human species took hundreds of thousands of years just to carve out a place for themselves in the world. They coexisted in relative balance but geographically separate from each other. And here came a few new cousins. Within 50,000 years, all other humanoid species were wiped out. Gone. Extinct. Homo sapiens alone dominate the world. The phenomenon is so extraordinary and out of proportion with previous history, no wonder it did not gain popularity until recently. It seemed more sensible if humans evolved on the time scale of a million years or so, as the fossils of other primates and humanoids suggest.
But the genomic evidence cannot be helped. Homo sapiens exploded onto the world with an exponential speed. Their routes of migration are illustrated on the National Geographic site The Human Journey.
Last weekend, my friend Helen made a profound observation about this subject: How did we not see this? Why couldn't we see that humans all over the world are absurdly alike, until the truth is forced on us? Looking back, the common traits are unmistakable. We all think abstractly and take to our symbols. We all have language. We organize society in nearly the same way. Despite cultural and historical differences, we instinctively understand each other like no other species. In "The World Until Yesterday," Jared Diamond described how remote islanders who had lived all their lives in primitive tribes could adapt to life in the 21st century within a just few years. And yet, we are naturally drawn to the theory that we separately evolved in multiple regions. Many Chinese academics still believe that the Chinese descended from Homo erectus pekinensis, whose fossils are dated to 500,000 years ago. Sorry to burst their bubbles, but the poor Homo erecus pekinensis were likely victims replaced by the ancestors of modern Chinese.
Why haven't humans preserved the history of leaving their original tribe behind or separating from their brothers and sisters and cousins? Without this tribal memory, when we encounter our long-lost cousins from another part of the world, all we want to do is to make war with them, rather than recognizing our shared lineage and hug each other. One possible explanation is that any history transmitted purely orally (before the invention of writing and means of recording writing) is fragile and prone to loss. But I think there may be something deeply embedded in human nature that drives us to believe in our own uniqueness. Funny how we are exactly the same in our narcissism and exceptionalism, which further supports the single origin theory. It is like what GK Chesterton pointed out, the two sides are in complete agreement, and therefore they go to war with each other.
Also ironic as hell is how we feel so lonely in this world that we set up massive telescopes and send out spacecrafts trying to find intelligent beings like us on other planets, even though we in fact lived alongside our cousins once upon a time and probably killed them all off. Perhaps all of these phenomena are manifestation of the same underlying trait --- we want to believe that we/I/our people are unique; we are perhaps the most aggressive and ruthless branch in archaic human species; when we encounter someone like us, we want to kill them or at least outcompete them rather than making friends; and last but not least, we are now the only human species left. We may not be inclined to the truth, but our bias may confer some kind of advantage. That is why truth is often hard to accept, even when it's right under our nose.
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