About three million years ago human predecessors embarked on a new course that would forever alter the evolution of our species. They became fierce predators, with a preference for red meat, a career change that brought greater exposure to deadly pathogens.
They not only survived, they thrived, a fact that has long bedeviled evolutionary biologists.
Why didn't that change in lifestyle kill them off? What could have given them the immunity from diseases carried by the animals they slaughtered and ate?
The key ingredient may have been sugar -- at least according to scientists at the University of California, San Diego, who have been searching for answers to these questions for many years. Or more precisely, the substitution of one sugar for another, which gave them the immunity they needed from some pathogens, but ironically led to massive deaths from another.
Sugar molecules are found on the surface of all animal cells, and they make interaction possible with other cells in the environment, including pathogens. For millions of years, the common ancestor of humans and apes shared a specific kind of sugar. Apes and chimps still have that type of sugar, but around three million years ago human ancestors substituted a slightly different sugar, according to the researchers.
That came about because of the mutation of a gene that is found in modern humans, but not in apes, our evolutionary cousins, the researchers reported earlier. The gene that causes the production of the sugar found on cells mutated sometime after our ancestors first stood upright, about 6 to 7 million years ago, and just as their brains began to expand in size, about 2.2 million years ago.
Medical professor Ajit Varki led a team that collected fossilized samples from around the world to see if they could pinpoint when that mutation occurred, launching our ancestors on a new trajectory. They found evidence of the mutated gene in Neanderthals more than half a million years ago, so it occurred before that. In their latest work, published in the Proceedings of the National Academy of Sciences, they conclude it probably happened around three million years ago. That would have been about the time of the emergence of Homo erectus, the likely ancestor of humans.
The researchers aren't certain why that change occurred, according to their latest study, published in the Proceedings of the National Academy of Sciences, but they suspect it had something to do with malaria.
About the time that our ancestors became hunters, the sugar switch occurred, making them immune to the parasite that causes malaria in apes, but that turned out to be a Faustian bargain. The new sugar made them vulnerable to a different malaria parasite that now accounts for more than a million human deaths every year.
"At about the same time, they started eating red meat, a major source of Neu5Gc (the sugar found on the cells of apes, but not humans) which may have further stimulated the immune response," evolutionary biologist Pascal Gagneux said in releasing the latest study. "It became viewed by their immune systems as foreign, something to be destroyed."
To test that concept, the researchers exposed chimpanzee sperm to human antibodies produced by the sugar, which found and killed the sperm in vitro.
That seemingly modest change - the substitution of one sugar for another - probably had a profound impact on pre-human evolution, Gagneux and colleagues believe. An enhanced immune system vastly improved the chances of successful reproduction, for example. It probably also caused them to disperse, according to one theory, because they would have distanced themselves from animals with infectious diseases.
Sugar, of course, is not the only factor at work here, the researchers say. It probably helped direct the course of human evolution, but there were many other directors as well. How we came to be what we are today still remains much of a mystery. But the researchers are reasonably confident they have added an important element.
"We suggest that the immune mechanism described here was involved in the origin of the genus Homo," the study concluded.