When it comes to evolution, people tend to focus on the big driving force of natural selection, which latches on to helpful mutations while purging the harmful ones. But there are other processes that change the frequencies of mutations—everything from random drift to the founding of small isolated populations.

Looking at our own species' history, we would expect to see some of this in action. After modern humans established themselves in Africa, smaller populations branched out to establish footholds in Asia before spreading east, eventually reaching the Americas. At each step, a small group of migrants took a fraction of humanity's genetic diversity with it, creating a series of population bottlenecks.

This should be easy to see in our DNA, but so far it has turned out to be complicated. Different attempts using distinct populations and methods have come to mixed conclusions about whether a clear signal is there. Now, a large international team of researchers has gone and sequenced genomes from multiple populations along humanity's route out of Africa, and they found a signature of these bottlenecks both in terms of genetic variation and in terms of potentially harmful mutations.

Humans have been present in Africa for much longer than anywhere else, and that shows up in our genetics. African populations tend to have the most genetic variability based on a number of measures. When some of that population left Africa about 50,000 years ago, they took only a part of that diversity with them into Asia. Some of that diversity went left into Europe, and a different subset took a right and spread through Asia, eventually reaching the Pacific, Australia, and the Americas.

The resulting populations haven't been around long enough for many new genetic changes to spread widely within them. And until recently, there hasn't been widespread mixing among the populations (with the exception of some regions like North Africa and the Mideast). So theoretically the genetic signature of the bottlenecks should be there.

Most attempts at trying to spot it, however, have focused on comparing European and African populations. Depending on the precise population used and the methods employed, they come up with different answers—typically, these studies have simply compared African-Americans to European-Americans, since these were the easiest populations to obtain DNA from.

The new study involves obtaining draft genomes from multiple populations along the migratory route out of Africa. These include some of the oldest human populations in Africa (the San and Mbuti), along with a North-African population, the Parthians of Central Asia, and natives of Cambodia, eastern Siberia, and Central America (Cambodians, Yakut, and Maya, respectively). Comparisons between these genomes were then analyzed according to their migratory distance from Africa.

Many of the variations in the human genome have no effect on fitness; they're not in or anywhere near genes, and changes there don't seem to affect anything. In some human populations, these locations can hold any of the four DNA bases. But as you move farther from Africa, more of these are likely to hold just one of them—apparently, the remaining diversity was lost in a bottleneck.

Things were even more dramatic when considering changes that occur within genes. These could be analyzed in light of a change's severity. Some mutations are likely to truncate the protein they encode, causing a severe effect; others will change its sequence of amino acids, causing a less severe problem.

Severe mutations were significantly more common in populations that were farther from our African origin, with the Mayan population having the most. In some cases, the populations had nothing but the harmful variant. Either natural selection simply hasn't had time to get rid of them, or those populations ended up starting out with nothing but the mutation.

The same was true when it comes to mutations with a moderate severity. These have been actively selected against in African populations but behave as if they're immune from selection outside of Africa.

This work is clearly consistent with the idea that humanity settled the globe through migrations of small populations that can be traced back to Africa, creating genetic bottlenecks along the way. But it does rely on some assumptions. The severity of most of the mutations was inferred through a computer analysis, which won't be accurate in every case. Still, I'd expect the results to hold up as we get more genomes from a greater diversity of populations in the future.

The more interesting complication to these results is that the African populations weren't quite as diverse as the computer analysis would have predicted. While it's easy to focus on the migration out of Africa, human populations have also migrated within Africa, and the continent has geography that can easily isolate peoples as well. It's likely that the picture of our African ancestry will get even more complicated as we begin to sample our genetic diversity more deeply there.

PNAS, 2015. DOI: 10.1073/pnas.1510805112 (About DOIs).