A Paleolithic Raw Bar, and the Human Brush With Extinction

Bret S. Stetka, MD

Disclosures

March 25, 2021

This essay is adapted from the newly released book A History of the Human Brain: From the Sea Sponge to CRISPR, How Our Brain Evolved.

"He was a bold man that first ate an oyster."

—Jonathan Swift

That man or, just as likely, that woman may have done so out of necessity. It was either eat this glistening, gray blob of briny goo or perish.

Beginning 190,000 years ago, a glacial age we identify today as Marine Isotope Stage 6, or MIS6, had set in, cooling and drying out much of the planet. There was widespread drought, leaving the African plains a harsher, more barren substrate for survival — an arena of competition, desperation, and starvation for many species, including ours. Some estimates have the Sapien population dipping to just a few hundred people during MIS6. Like other apes today, we were an endangered species. But through some nexus of intelligence, ecological exploitation, and luck, we managed. Anthropologists argue over what part of Africa would've been hospitable enough to rescue Homo sapiens from Darwinian oblivion. Arizona State University archeologist Curtis Marean believes the continent's southern shore is a good candidate.

For two decades, Marean has overseen excavations at a site called Pinnacle Point on the South African coast. The region has over 9000 plant species, including the world's most diverse population of geophytes, plants with underground energy-storage organs like bulbs, tubers, and rhizomes. These subterranean stores are rich in calories and carbohydrates, and, by virtue of being buried, are protected from most other species (save the occasional tool-wielding chimpanzee). They are also adapted to cold climates and, when cooked, easily digested. All in all, a coup for hunter-gatherers.

The other enticement at Pinnacle Point could be found with a few easy steps toward the sea. Mollusks. Geological samples from MIS6 show South Africa's shores were packed with mussels, oysters, clams, and a variety of sea snails. We almost certainly turned to them for nutrition.

Marean's research suggests that, sometime around 160,000 years ago, at least one group of Sapiens began supplementing their terrestrial diet by exploiting the region's rich shellfish beds. This is the oldest evidence to date of humans consistently feasting on seafood — easy, predictable, immobile calories. No hunting required. As inland Africa dried up, learning to shuck mussels and oysters was a key adaptation to coastal living, one that supported our later migration out of the continent.

Marean believes the change in behavior was possible thanks to our already keen brains, which supported an ability to track tides, especially spring tides. Spring tides occur twice a month with each new and full moon and result in the greatest difference between high and low tidewaters. The people of Pinnacle Point learned to exploit this cycle. "By tracking tides, we would have had easy, reliable access to high-quality proteins and fats from shellfish every two weeks as the ocean receded," he says. "Whereas you can't rely on land animals to always be in the same place at the same time." Work by Jan De Vynck, a professor at Nelson Mandela University in South Africa, supports this idea, showing that foraging shellfish beds under optimal tidal conditions can yield a staggering 3500 calories per hour!

"I don't know if we owe our existence to seafood, but it was certainly important for the population Curtis studies. That place is full of mussels," says Ian Tattersall, curator emeritus with the American Museum of Natural History in New York City, New York.

"And I like the idea that during a population bottleneck we got creative and learned how to focus on marine resources." Innovations, Tattersall explains, typically occur in small, fixed populations. Large populations have too much genetic inertia to support radical innovation; the status quo is enough to survive. "If you're looking for evolutionary innovation, you have to look at smaller groups."

MIS6 wasn't the only near-extinction in our past. During the Pleistocene epoch, roughly 2.5 million to 12,000 years ago, humans tended to maintain a small population, hovering around a million and later growing to maybe eight million at most. Periodically, our numbers dipped as climate shifts, natural disasters, and food shortages brought us dangerously close to extinction. Modern humans are descended from the hearty survivors of these bottlenecks.

One especially dire stretch occurred around one million years ago. Our effective population (the number of breeding individuals) shriveled to around 18,000, smaller than that of other apes at the time. Worse, our genetic diversity — the insurance policy on evolutionary success and the ability to adapt — plummeted. A similar near extinction may have occurred around 75,000 years ago, the result of a massive volcanic eruption in Sumatra.

Our smarts and adaptability helped us endure these tough times — omnivorism helped us weather scarcity.

A Sea of Vitamins

Both Marean and Tattersall agree that the Sapiens hanging on in southern Africa couldn't have lived entirely on shellfish.

Most likely they also spent time hunting and foraging roots inland, making pilgrimages to the sea during spring tides. Marean believes coastal cuisine may have allowed a paltry human population to hang on until climate change led to more hospitable terrain. He's not entirely sold on the idea that marine life was necessarily a driver of human brain evolution.

By the time we incorporated seafood into our diets we were already smart, our brains shaped through millennia of selection for intelligence. "Being a marine forager requires a certain degree of sophisticated smarts," he says. It requires tracking the lunar cycle and planning excursions to the coast at the right times. Shellfish were simply another source of calories.

Unless you ask Michael Crawford.

Crawford is a professor at Imperial College London and a strident believer that our brains are those of sea creatures. Sort of.

In 1972, he co-published a paper concluding that the brain is structurally and functionally dependent on an omega-3 fatty acid called docosahexaenoic acid, or DHA. The human brain is composed of nearly 60% fat, so it's not surprising that certain fats are important to brain health. Nearly 50 years after Crawford's study, omega-3 supplements are now a multi-billion-dollar business.

Omega-3s, or more formally, omega-3 polyunsaturated fatty acids (PUFAs), are essential fats, meaning they aren't produced by the body and must be obtained through diet. We get them from vegetable oils, nuts, seeds, and animals that eat such things. But take an informal poll, and you'll find most people probably associate omega fatty acids with fish and other seafood.

The animal brain evolved 600 million years ago in the ocean and was dependent on DHA…

In the 1970s and 1980s, scientists took notice of the low rates of heart disease in Eskimo communities. Research linked their cardiovascular health to a high-fish diet (though fish cannot produce omega-3s, they source them from algae), and eventually the medical and scientific communities began to rethink fat. Study after study found omega-3 fatty acids to be healthy. They were linked with a lower risk for heart disease and overall mortality. All those decades of parents forcing various fish oils on their grimacing children now had some science behind them. There is such a thing as a good fat.

Recent studies show that some of omega-3s' purported health benefits were exaggerated, but they do appear to benefit the brain, especially DHA and eicosapentaenoic acid, or EPA. Omega fats provide structure to neuronal cell membranes and are crucial in neuron-to-neuron communication. They increase levels of a protein called BDNF, which supports neuronal growth and survival. A growing body of evidence shows omega-3 supplementation may slow down the process of neurodegeneration, the gradual deterioration of the brain that results in Alzheimer's disease and other forms of dementia.

Popping a daily omega-3 supplement or, better still, eating a seafood-rich diet, may increase blood flow to the brain. In 2019, the International Society for Nutritional Psychiatry Research recommended omega-3s as an adjunct therapy for major depressive disorder. PUFAs appear to reduce the risk for and severity of mood disorders like depression and to boost attention in children with ADHD as effectively as drug therapies.

Many researchers claim there would've been plenty of DHA available on land to support early humans, and marine foods were just one of many sources.

Not Crawford.

He believes that brain development and function are not only dependent on DHA but, in fact, DHA sourced from the sea was critical to mammalian brain evolution. "The animal brain evolved 600 million years ago in the ocean and was dependent on DHA, as well as compounds such as iodine, which is also in short supply on land," he says. "To build a brain, you need these building blocks, which were rich at sea and on rocky shores."

Crawford cites his early biochemical work showing DHA isn't readily accessible from the muscle tissue of land animals. Using DHA tagged with a radioactive isotope, he and his colleagues in the 1970s found that "ready-made" DHA, like that found in shellfish, is incorporated into the developing rat brain with 10-fold greater efficiency than plant- and land animal–sourced DHA, where it exists as its metabolic precursor alpha-linoleic acid. "I'm afraid the idea that ample DHA was available from the fats of animals on the savanna is just not true," he disputes. According to Crawford, our tiny, wormlike ancestors were able to evolve primitive nervous systems and flit through the silt thanks to the abundance of healthy fat to be had by living in the ocean and consuming algae.

For over 40 years, Crawford has argued that rising rates of mental illness are a result of post–World War II dietary changes, especially the move toward land-sourced food and the medical community's subsequent support of low-fat diets. He feels that omega-3s from seafood were critical to humans' rapid neural march toward higher cognition, and are therefore critical to brain health. "The continued rise in mental illness is an incredibly important threat to mankind and society, and moving away from marine foods is a major contributor," says Crawford.

University of Sherbrooke physiology professor Stephen Cunnane tends to agree that aquatically sourced nutrients were crucial to human evolution. It's the importance of coastal living he's not sure about. He believes hominins would've incorporated fish from lakes and rivers into their diet for millions of years. In his view, it wasn't just omega-3s that contributed to our big brains, but a cluster of nutrients found in fish: iodine, iron, zinc, copper, and selenium among them. "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," he says. "Numerous other nutrients found in fish and shellfish were very probably important too and are now known to be good for the brain."

Marean agrees. "Accessing the marine food chain could have had a huge impact on fertility, survival, and overall health, including brain health, in part, due to the high return on omega-3 fatty acids and other nutrients." But, he speculates, before MIS6, hominins would have had access to plenty of brain-healthy terrestrial nutrition, including meat from animals that consumed omega-3-rich plants and grains.

Cunnane agrees with Marean to a degree. He's confident that higher intelligence evolved gradually over millions of years as mutations inching the cognitive needle forward conferred survival and reproductive advantages — but he maintains that certain advantages like, say, being able to shuck an oyster, allowed an already intelligent brain to thrive.

Foraging marine life in the waters off of Africa likely played an important role in keeping some of our ancestors alive and supported our subsequent propagation throughout the world. By this point, the human brain was already a marvel of consciousness and computing, not too dissimilar to the one we carry around today.

In all likelihood, Pleistocene humans probably got their nutrients and calories wherever they could. If we lived inland, we hunted. Maybe we speared the occasional catfish. We sourced nutrients from fruits, leaves, and nuts. A few times a month, those of us near the coast enjoyed a feast of mussels and oysters.

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