by Craig Weatherby
Our earliest human-like ancestors evolved brains bigger than their primate cousins'.
Was it because they gravitated to watery shores, abundant in plants and animals high in DHA, the long-chain “marine” omega-3 that dominates human brains?
When we attended the 2005 Seafood & Health conference in Washington DC, we were greatly impressed with a keynote speech by Professor Michael Crawford, Ph.D. director of the Institute of Brain Chemistry and Human Nutrition at North London University.
However, Dr. Crawford's hypothesis has many critics, who note that modern humans thrive on diets rather low in DHA, and that many pre-human and early human species roamed areas not known for abundant aquatic life. In addition, there is no evidence a diet high in DHA alone results bigger brains, as attested by the tiny brains of fish and seabirds.
Critics of the water-foods hypothesis of brain evolution also point to the proven association between proportionally bigger brains and diverse, high-fat, and high-protein diets in general.
While neither side in this debate is likely to be proven completely right or wrong, new evidence seems to tip the scales toward Dr. Crawford's camp.
Earliest-ever seashore settlements found on South African coast
Most researchers believe that modern humans first emerged between 150,000 and 200,000 years ago, in eastern Africa.
However, it is not clear exactly when early humans developed the capacity for symbolic thought, such as language and symbolic or representative art.
The debate between advocates of Dr. Crawford's “aquatic brain” hypothesis and the skeptical majority of paleoanthropologists may rest on the answer to a key question.
Did modern cognitive abilities occurred gradually, purely in response to natural selection, or quite suddenly, after a boost in brain size and/or neurological capacity that was fueled by hominid diets newly rich in aquatic foods?
The prestigious journal Nature just published findings from a team led by paleoanthropologist Curtis Marean of Arizona State University … discoveries that seem to support the possibility of a diet-driven burst in brain size or power (ASU 2007).
Mareans' team found evidence of early humans living on the coast in South Africa 164,000 years ago: a date far earlier than any ever documented for shore-dwelling humans, and smack in the center of the time period when anatomically modern humans emerged.
(By “anatomically modern”, paleoanthropologists mean homo-genus primates highly similar to 21st century Homo sapiens in terms of body proportions, teeth, jaws, skull shape, brain-cavity size, and other major features.)
As Dr. Marean said in a press release, “Our findings show that at 164,000 years ago in coastal South Africa humans expanded their diet to include shellfish and other marine resources, perhaps as a response to harsh environmental conditions. This is the earliest dated observation of this behavior.” (ASU 2007)
Critically, Marean's team discovered evidence that these people were using pigment, probably to make symbols or art, and were using “bladelet” stone tool technology, whose oldest previously documented occurrence had been about 70,000 years ago.
The ASU release makes a very pertinent point: “These new findings not only move back the timeline for the evolution of modern humans, they show that lifestyles focused on coastal habitats and resources may have been crucial to the evolution and survival of these early humans” (ASU 2007).
These intriguing findings will not end the debate over brain evolution, but they will certainly keep it nourished.
And other recent research lends support—albeit less direct—to the “aquatic brain” hypothesis of evolution.
Pre-human savanna dwellers show evidence of shellfish diets
Two years ago, it was reported that instead of the exclusively plant-based diet that savanna-dwelling pre-humans were thought to eat, they might have been omnivores who ate fresh-water snails and crustaceans.
The earliest members of our genus, Homo, shared the African plains with another group of hominids, the australopithecines, for the first two million years of their existence. The two groups were closely related, but the australopithecines had giant molars, thick tooth enamel and a bony skull crest that anchored huge chewing muscles.
Paleoanthropologists presumed, logically enough, that the australopithecines used their heavy duty head and jaw structures to chew tough plant foods. But the results of a 2005 study suggested otherwise.
Homo developed a large brain and tool-making capabilities that enabled it to pursue a diet rich in meat, and researchers have presumed that the australopithecines evolved rugged anatomical equipment to grind up tough plant fare.
But recent findings from studies of carbon isotopes in ancestral African fossils show that the ratio of carbon 13 to carbon 12 in the teeth of australopithecines is higher than the ratio seen in animals that eat mostly fruit and nuts, but lower than in animals that subsist on grass seeds (Wong K 2006).
Alan B. Shabel of the University of California, Berkeley, has hypothesized that australopithecines' teeth and jaws were built for eating hard-shelled invertebrates like snails and shellfish.
Even though eastern and southern Africa—where virtually all of the continent's hominid and australopithecine fossils are found—was in a dry period when the two pre-human species co-existed, wetlands dotted these savannas and plains than as they do now.
Much like modern wetlands, these ancient lakes, rivers and marshes were rich in shoreline creatures such as giant land snails and crabs.
And when Shabel examined modem African otters and the marsh mongooses, which specialize in eating these very same snails and crabs, he found the same distinctive skull features seen in the australopithecines.
Shabel then analyzed carbon isotopes in the otters and mongoose and their snail and crab prey. Indeed, the carbon profiles of all of these animals' were similar to those of the ancient pre-human australopithecines (Wong K 2006).
This does not provide direct evidence for the idea that modern human brains were made possible by eating aquatic foods.
But it does suggest that savanna-dwelling pre-humans had access to aquatic foods, and ate significant amounts of them.
Our more direct Homo-genus ancestors did not evolve big jaws to chew shells. Instead, their brains are believed to have been bigger than the australopithecines', so Homo species may have smashed aquatic animals' shells or used tools to dig out the meat.
Thus, absent big teeth and jaws our Homo predecessors could still make protein- and omega-3-rich snails and crabs part of their omnivorous diet, thereby enabling faster brain evolution.
And the new findings out of South Africa suggest that availability of a far greater abundance of omega-3s, from ocean animals and plants, may have fueled a rapid leap forward in human brain size and power.
We'll keep you posted on this intriguing, still-unfolding area of evolutionary inquiry.
- Arizona State University (ASU). ASU team detects earliest modern humans. Accessed online October 20, 2007 at http://asunews.asu.edu/20071016_eatrlyhumans
- Wong K. Food for Thought: Giant Hominid teeth not for crunching nuts, but shellfish. Scientific American, February 13, 2006. Accessed online October 20, 2007 at http://www.sciam.com/article.cfm?articleID=000EA7AB-FCE4-13CB-BC1C83414B7F012A&sc=I100322
- Minkel JR. Earliest Known Seafood Dinner Discovered: Dished out with a side of symbolic thought. Scientific American, October 17, 2007. Accessed online October 20, 2007 at http://www.sciam.com/article.cfm?articleID=AFEAEA17-AE57-F733-97E39F5BE72928D2&chanID=sa027