DNA Change Allowed Ancient Humans to Escape Africa
Humans Could Migrate Because They Were No Longer Dependent on Fish
A new study out suggests that humans were able to spread out and take over the world because of a new DNA change (or mutation) that popped up in their DNA 85,000 or so years ago. This DNA change didn’t make humans suddenly crave new adventures. Instead it let them get away with just eating plants for good brain development.
This was enough to unleash humans onto the world. Nothing has been the same since this tiny mistake first appeared in our three billion letters of DNA.
Mutating Away from the Water
Human brains are sort of like modern electronics – they need rare materials to have them work as well as they should. While an iPhone needs various rare Earth minerals, human brains need lots of something called long chain polyunsaturated fatty acids or LC-PUFAs.
Humans are terrible at making these fatty acids on their own and so have to get them from their diet (sort of like vitamin C). Unfortunately for our ancestors, humans aren’t very good at turning a plant’s fatty acids into the ones they need. And even though we are good at getting them from animals, we didn’t start hunting in a big way until around 50,000 years ago.
Human ancestors probably got these fatty acids from fish and other aquatic animals. In fact, scientists have long hypothesized that our ancestors’ need for aquatic animals was a big reason humans were such stick-in-the-muds for almost 100,000 years. Their inability to get the brain food they needed from plants kept them trapped by the water’s edge, unable to spread across the globe.
And yet, humans did start to spread in earnest across Africa around 60-80,000 years ago. This is thousands of years before there is any evidence that they did a lot of hunting of land animals. Clearly something changed sometime just before these humans started moving away from the shore.
A group of researchers thinks that the answer might lie in a mutation that allowed ancient humans to better use plant fatty acids to make the LC-PUFAs they needed for their brains. Now humans could eat plants to get enough nutrients and so could migrate and conquer the world.
The evidence to support this idea is the fact that Africans have a certain DNA difference that Asians and Europeans do not. This difference strengthens an enzyme (FADS1) that converts plant fatty acids into the ones humans need for their brains. In other words, Africans have a mutation that allows them to get at least some of their brain fatty acids from plants.
When the researchers looked at the DNA surrounding this enzyme in Africans, they saw fewer DNA differences than expected. This is a telltale sign that once the mutation appeared, it quickly spread through the population (a “selective sweep” in genetics lingo). So once a few people were able to use plants effectively, they were off and running and quickly populated the world.
A close look at the DNA also allowed these scientists to estimate that the mutation first appeared around 85,000 years ago (which fits the story nicely). Unfortunately these analyses are not very precise and there is a wide range of possible dates centered around 85,000 years ago. The actual number from the paper is 85,000 +/- 84,000 years ago.
The researchers also think that this DNA difference doesn’t come without a cost. They conclude this from the fact that the mutation is no longer than common in Asians and Europeans. The idea is that once humans could hunt for land animals, they no longer needed to rely on plants and so the mutation was lost.
Of course another possibility is that the humans that headed for Europe and Asia just happened not to have this DNA difference any more. Humans didn’t leave Africa until 40,000 years ago, well after they had started hunting and so could get their fatty acids from the animals they were eating. Figuring out whether Europeans and Asians lost the DNA difference or never had it in the first place will have to wait for more studies.
As scientists sequence more and more humans (and their long lost relatives), they will learn more and more about human evolutionary history. They will begin to piece together why humans moved the way they did, who they bred with before wiping them out and much, much more. Ultimately they may be able to explain what about our DNA makes us human.
By Dr. Barry Starr, Stanford University