The reason you (probably - let's not make assumptions here) don't live by an African lake and feed mainly on fish may be a single, ancient mutation.
Homo sapiens appeared 180,000 years ago, but for the next hundred thousand years stuck to a comparatively small area centered around a number of central African lakes.
After this time, though, the species moved out from central Africa and across the continent in what has been called the great expansion - ultimately ending up in your living room.
The reason, say Wake Forest Baptist Medical Center researchers, is the appearance more than 85,000 years ago of a genetic variant in a key gene cluster on chromosome 11, known as the fatty acid desaturase cluster.
This variation would for the first time have allowed early humans to tuck into their veg with some purpose, by converting plant-based polyunsaturated fatty acids (PUFAs) to brain PUFAs - necessary for increased brain size, complexity and function.
The findings of the genetic analysis may explain why early humans hung around their lakes for so long. It implies that to support complex brain function, they needed large amounts of the long-chain PUFA docosahexaenoic acid (DHA) - found in shellfish and fish. And this would have been a particularly important foodstuff at the time, as organized hunting hadn't yet developed.
"This may have kept early humans tethered to the water in central Africa where there was a constant food source of DHA," says professor Floyd Chilton.
"There has been considerable debate on how early humans were able to obtain sufficient DHA necessary to maintain brain size and complexity. It's amazing to think we may have uncovered the region of genetic variation that arose about the time that early humans moved out of this central region in what has been called the great expansion."
Once this trait arose, says the team, it was under intense selective pressure and thus spread rapidly throughout the entire African population.
The discovery supports a finding by Chilton that people of African descent are ore likely to carry the gene variants that convert the plant-based medium-chain omega-6 PUFAs found in cooking oils and processed foods to the long-chain PUFAs that cause inflammation. It's why African Americans have much higher rates of hypertension, type 2 diabetes, stroke, coronary heart disease and certain types of cancer than Caucasians.
"The current observation provides another important clue as to why diverse racial and ethnic populations likely respond differently to the modern western diet," says Chilton.