Modern humans and their societies reflect what their ancestors ate, researchers say. "Diet controls everything – locomotion, social organization, and reproduction," says anthropologist Nathaniel Dominy.
As scientists pursue that thought, they find new reasons to believe that diet is a key factor in the evolution of human physiology over the past several million years and of human urban societies over the past several thousand years.
The scientists' conclusions are speculative. Yet, where scientific evidence can inform them, they are gaining a harder edge.
John Day at Louisiana State University in Baton Rouge and several colleagues find enlightenment in geophysics. In an essay published last month in the Transactions of the American Geophysical Union, they link the natural global warming that followed the last ice age to the rise of urban civilization. Earth's global average temperature has risen 6 degrees C (11 degrees F.) in the past 10,000 years. Sea level has come up some 120 meters. Most of the rise occurred rapidly during the first 3,000 years. Then sea level became relatively stable. It has risen only a few meters in the past 7,000 years.
The scientists explain that the early rapid inundation would have established a new coastal habitat for humans. Changes such as water depths and nutrient availability in the near-shore environment and in river estuaries would have created an abundance of marine life. This development would have favored a settled environment and encouraged urbanization with class distinctions and the ability to undertake major public construction projects.
Dr. Day and colleagues conclude that the rich marine food source "underwrote the considerable social costs of creating these class societies, thus establishing the key element of urban lifestyles." Even when new agriculture-based cities developed inland, inhabitants imported a lot of seafood. Thus, the scientists say, sea level stabilization "opened the gates to civilization."
Nathaniel Dominy at the University of California in Santa Cruz is part of a group that's looking much deeper into the past with the help of isotopic chemistry. An element such as carbon has several slightly different forms called isotopes. The relative abundance of certain isotopes in tooth enamel in animals and humans reflect the foods the teeth processed.
Two million years ago, human ancestors, called hominins, had teeth that, like our own, are shaped to process seeds and nuts. Yet isotopic analysis of the hominins' tooth enamel showed signs of grasses and sedges. That's tough stuff to chew.
Did our ancestors really munch on it?
Dr. Dominy's group thinks they were eating a much more nutritious and easily chewed diet of tubers, bulbs, and other underground parts of plants. The scientists studied African mole rats that have a similar diet. Modern mole-rat teeth show that this diet can leave an isotopic signature similar to that left by eating grasses and sedges. Fossil mole-rat teeth from the sites where hominin fossils were found also have this signature.
Dominy explains that it is logical to conclude that our hominin ancestors had a far richer diet than grass. He adds: "Something about our ancestors' diet shifted to favor them becoming bipedal and increasingly brainy. No other organism on the planet evolved that way."