Where do you live? I live in eastern Massachusetts, at least for now. Where do you and your family come from? I’m from Parkersburg, West Virginia. My paternal family settled in present-day West Virginia around the time of the American Revolutionary War – about 1780. Where are your ancestors from? Mine come mostly from all over present-day Germany, with some others spread out across the British Isles. But what about your ancestors’ ancestors? Where did they come from?
“Trick question,” you say. “How can I possibly know that if they didn’t have written records back then?”
Ah, but you do know. You know because the answer is the same for all 6.8 billion-and-change humans on this planet: Africa. All modern humans can trace their origins to one or maybe (but probably not) two distinct relatively small population(s) in Africa somewhere around 100,000 to 200,000 years ago. So all of us, initially, as humans, came from Africa.
We know this from fossils and, as I’ve discussed in the above link, evidence from mitochondrial DNA, both of which match up quite well. However, until recently no one has looked at nuclear DNA to scan for signs of how old the indigenous populations there are. Simply put, the older a population is, the more mutations it should carry in it collective genome compared to others, as mutations arise naturally and at a relatively predictable rate over generations.
A group of scientists spread across several continents did just that, publishing in a
Though the four Bushmen lived in separate tribes and spoke different languages, they were thought to represent a good model for studying the original progenitor population from which all humans arose. By examining the amount of variation within the Bushmen tribes, these scientists could confirm just how ancient their population is. You might recognize the fifth man on the bottom left above as Archbishop Desmond Tutu of South Africa. As Tutu is of Bantu – not Khoisan – ancestry, his genome was used as a sort of “control” to test for the level of variation within different modern African populations. These all, in turn, were compared to previously-described genomes of direct European ancestry (eg, that of DNA pioneer James Watson) to get a good picture of the level of variation within African populations thought to be unmoved by global migration since the dawn of Homo sapiens.
In looking for differences in genomes, geneticists look for what’s termed SNPs, for “single nucleotide polymorphisms.” Basically, SNPs are places where one single base-pair in a genetic sequence differs from that in another genetic sequence. As these are mutations that can arise over generations, counting the numbers of SNPs within a population is a way of determining just how old that population is. Not only did the researchers on this study find about 1.3 million SNPs in !Gubi’s genome (out of a total of about 3 billion base pairs in the human genome) that had not been found in previous sequences from more direct European and Asian descent, they also found that, based on the analysis, there was roughly as much genetic variation within the Khoisan population as there is between an average European and an average Asian. This huge amount of genetic variation proved, once again, that native Bushmen populations in Africa are incredibly ancient and can trace their geographically-stable roots back further than any population from anywhere else on the globe.
One thing you might be wondering now is, “then why don’t they look more different than one another?” Or something similar in an attempt to point out the obvious that these Bushmen look much more like one another than a man from Germany and a man from China might look in comparison – all while striving to not sound stupendously racist in the process. It has to do with the fact that this genotypic variation does not necessarily translate to phenotypic variation: a change in the code doesn’t mean a change in the output. Much of our DNA simply isn’t translated into proteins, and even if it is, single amino-acid changes don’t necessarily have a massive change on the overall function of the protein (and when they do the change is most always deleterious and therefore selected-against). Furthermore, different changes might result in similar phenotypes given the right selective pressure, so, just because two individuals are genetically different doesn’t mean they’re going to look quite different, as is the case here.
The conclusion that can be gathered from this study is that wide genome sequencing of members of different populations should be able to help us to determine in the future a more thorough picture of human ancestry. Since these nuclear genome results agree with the hypothesis based on archaeological and mitochondrial DNA that there should be more variance within native African populations than in any other broad ethnic groups, genetics can be used as a tool in the future to make more precise determinations of the lineages of individuals. With enough samples, eventually a sufficient database of SNPs can be generated and compared with known ancestry to be able to give just about any human a near-complete history of the global migrations of his or her ancestors over the past 100,000 years. And that’s one heck of a family tree.
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March 23rd, 2010 at 10:48 am
[...] We’re pretty cool, right? We know that we emerged out of a relatively small population in Africa around 200,000 years ago and have since gone on to expand our numbers to nearly 7 billion and [...]