Omega fatty acids, including docosahexaenoic acid, or DHA, are key to brain health and most likely helped to drive the evolution of the modern human brain. But how did early humans access these vital nutrients? The answer is a matter of some debate.
For nearly two decades archaeologist Curtis W. Marean, associate director of Arizona State University's Institute of Human Origins, has overseen excavations at a site called Pinnacle Point on South Africa's southern coast, near where a newly discovered early human species, Homo naledi, was recently unearthed. His work there suggests that sometime between 195,000 and 123,000 years ago, during a glacial period known as Marine Isotope Stage 6 (MIS6), humans made a significant shift in their eating habits, moving from foraging for terrestrial plants, animals and the occasional inland fish to relying on the rich, predictable shellfish beds in the area.
Marean believes that this change occurred when early humans learned to exploit the bimonthly spring tides. And to do so, he says, our brains were already fairly well evolved. “Accessing the marine food chain could have had huge impacts on fertility, survival and overall health, including brain health,” Marean explains, in part because of the high return on omega-3 fatty acids. But before MIS6, he speculates, hominins would have had access to plenty of brain-healthy terrestrial nutrition, including by feeding on animals that consumed omega-3-rich plants and grains.
Others disagree, at least in part. “I'm afraid the idea that ample DHA was available from the fats of animals on the savanna is just not true,” says psychiatrist Michael A. Crawford of Imperial College London. “The animal brain evolved 600 million years ago in the ocean and was dependent on DHA and compounds essential to the brain such as iodine, which is also in short supply on land. To build a brain, you would need building blocks that were rich at sea and on rocky shores.”
Crawford's early biochemical work focused on showing that DHA is not readily accessible from the muscle tissue of land animals. Using DHA tagged with a radioactive isotope, he and his colleagues also demonstrated that “ready-made” DHA—such as that found in shellfish—is incorporated into the developing rat brain with 10-fold greater efficiency than plant-sourced DHA.
Crawford's colleague and collaborator, physiologist Stephen Cunnane of the University of Sherbrooke in Quebec, also feels that aquatically sourced food was crucial to human evolution. But he believes that before MIS6, inland hominins had already incorporated fish from lakes and rivers into their diet for millions of years.
He suggests that it was not just omega-3s but a cluster of nutrients found in fish (including iodine, iron, zinc, copper and selenium) that contributed to our big brain. “I think DHA was hugely important to our evolution and brain health, but I don't think it was a magic bullet all by itself,” Crawford says.
All three researchers are confident that higher intelligence evolved gradually over millions of years as mutations inched the cognitive needle forward, conferring survival and reproductive advantages. But advantages such as, say, figuring out how to shuck oysters—as well as track the spring tides—threw open the Darwinian floodgates. Cunnane comments: “Once we were able to access the coastal food chain in Africa—far more rich and reliable than inland sources of fish—brain and cultural evolution exploded.”