Really good fights in science rarely get settled. One side will advance evidence that the other side is wrong, only to have the “wrong” side respond with evidence proving the reverse. Slugging it out, back and forth, a clearer picture gradually emerges.
Once in a while, however, somebody actually wins. Definitive “no-doubt-about-it” evidence emerges proving that one idea is correct and the opposing view simply wrong.
Just such a clear resolution has happened this year to a particularly angry and bitter battle that has raged for decades among scientists who study mankind’s origins.
The Competing Ideas. One group of scientists argue that our species, Homo sapiens, is a late arrival on the human scene. The recent African origin hypothesis proposes that H. sapiens evolved in a single location in central Africa about 200,000 years ago and migrated out of Africa relatively recently. As our species spread across Asia and later Europe, it replaced the local populations of more primitive species of humans like Homo erectus that had arrived from Africa in earlier migrations.
The opposing group of scientists argue that our species evolved not in one place, but in multiple locations. The multiregional hypothesis proposes that H. sapiens evolved from the earlier-arriving human species like H. erectus more or less simultaneously in Africa, Asia, and Europe. Occasional migration would have kept the populations evolving in concert, they argue, different strands of a braided evolutionary rope.
Testing the hypotheses. As the scientific battle has raged, both sides could point to evidence supporting their positions, but because all of the evidence was indirect, no one was convinced who didn’t want to be. This fall, the rival hypotheses have been tested directly, not once but twice. Each test leads to the same conclusion: multiregionalism loses the battle.
1. The Y chromosome family tree. The first nail in the coffin of multiregionalism came last month, with publication in the American Journal of Human Genetics of a multi-national investigation into the human family tree.
Human family trees are not easy to make, because most human genes trade bits with each other every generation. When eggs and sperm are made, pairs of chromosomes exchange genes like decks of cards being shuffled. This process, called genetic recombination, jumbles everything up. This mixing makes it impossible for a researcher to trace how a particular gene changes over time, like trying to follow one bee in a swarm.
A neat way out of this — a way to track a single bee — is to look at gene differences on the Y chromosome, which does not undergo recombination. Y chromosomes just pass down unchanged in males from one generation to the next. Any new changes that arise during evolution are easy
to track, as they too pass down. By looking at genes on the Y chromosome, you can build a clear family tree.
The researchers in this large study looked at the pattern of gene variation among a total of 1002 different Y chromosomes from European males. While they found many different patterns of variation, fully 80% of European males shared a single pattern, suggesting modern Europeans had a common ancestor. The data indicate the pattern arose some 40,000 years ago. In other words, our species came to Europe recently. This result is just what the “recent African origin” hypothesis predicts.
2. The mDNA family tree. The second nail in the coffin of multiregionalism came with a report in the journal Nature last week. A Swedish and German research team set out to analyze the human family tree using another set of genes that does not undergo genetic recombination. Mitochondrial DNA (mDNA) is passed down from mother to daughter without recombination. Previous efforts to make human family trees using mDNA data have only considered one portion, called the “control region” (its easy to study). But because the control region is only 7% of the total sequence, results had weak statistical significance, and nobody much believed the mDNA human family trees.
These researchers overcame this difficulty by sequencing the entire mDNA from 53 individuals of differing ethnic background from Africa, Europe, Asia, and America. They found that all modern humans share a common ancestor 170,000 years ago, consistent with a large body of other data that H. sapiens originated in Africa at about that time.
The key finding of this study is that there is a distinct branch on the family tree 52,000 years ago, separating Africans from non-Africans. Here is the “no-doubt-about-it” evidence, clear for all to see: our species left Africa 52,000 years ago, just as the “recent African origin hypothesis” says. By this evidence, the multiregional hypothesis is clearly wrong.
So, now that the “proof” is in, is the war over? I doubt it. Scientists are by their nature a feisty bunch, and I would be surprised indeed if the multiregionalists take this lying down.
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