When Humans Almost Went Extinct
Around 1.2 million years ago, only 18,500 early humans were breeding on the planet. According to researchers, this is evidence that there was a real risk of extinction for our early ancestors. What's more, according to a new study it took at least a million years for humans to come back from the brink. It was not until the emergence of modern humans, Homo sapiens, around 160,000 years ago and their migration out of Africa that humanity's place on Earth was secured. Two factors helped humans to survive: an increasingly carnivorous diet and mastery of fire.
In an on online report in the Proceedings of the National Academy of Sciences (PNAS), “Mobile elements reveal small population size in the ancient ancestors of Homo sapiens,” researchers found that the ancient human effective population size 1.2 million years ago was about 18,500, and couldn't have been larger than 26,000. This means that the population of Homo erectus, an ancestor of modern humans, was small even at a time that the species was spreading around the world. This implies an “unusually small population size for a species spread across the entire Old World,” the authors write. “There's this history of a precarious existence not just for our species but for our ancestors,” says co-author Lynn Jorde, a human geneticist at the University of Utah in Salt Lake City.
On of the things that has long puzzled researchers is that modern humans lack the genetic variation found in other living primates. Compared with chimpanzees or gorillas, human genetic variation is remarkably small, even though our current population is so much larger than any species of great ape. One explanation for this lack of variation is that our species experienced events where a significant part of the human population were killed. Some researchers proposed that the lack of variation in our maternally inherited mitochondrial DNA suggested these events took place relatively recently, perhaps as our ancestors were migrating out of Africa.
Early humans migrated from Africa.
One possible indecent was the eruption of the Toba super-volcano, 70,000 years ago in Indonesia. Some scientists have speculated that the eruption could have triggered a “nuclear” winter that fewer than 15,000 individuals survived. Other genetic studies attributed our low genetic variation to chronically low numbers, with as few as 10,000 breeding humans at different times during the past 2 million years. To this point there have been technical problems with all these studies, making their conclusions uncertain.
For the PNAS study, a new method of studying markers across the entire genome is allowing geneticists to look back in time, to before the emergence of our species 200,000 years ago. Jorde and his colleagues used short lengths of DNA that randomly insert themselves into the genome, known as Alus, as probes to find ancient parts of the genome. This enabled the researchers to chart the population history of our really ancient ancestors, such as Homo erectus. According to the authors:
By comparing the likelihood of various demographic models, we estimate that the effective population size of human ancestors living before 1.2 million years ago was 18,500, and we can reject all models where the ancient effective population size was larger than 26,000. This result implies an unusually small population for a species spread across the entire Old World, particularly in light of the effective population sizes of chimpanzees (21,000) and gorillas (25,000), which each inhabit only one part of a single continent.
It is amazing to think how such a tiny population of humans managed to spread across the face of this planet, until today H. sapiens can be found living on every continent under all sorts of environmental conditions. From Arctic cold to tropical heat, from sodden rainforests to arid deserts, from sea level to mountain heights humans are literally everywhere on Earth. Humans spread across the world before civilization, before human technology rose above chipped flint knives and pointy sticks. What explains our ancestors' success?
Lunch was not easy to come by for our ancestors.
When viewed as wild animals, humans are singularly unimpressive. Poorly insulated, being mostly bereft of fur or hair, and noticeably weak—even an immature chimpanzee is stronger than an adult human. Our nails and teeth seem particularly unimpressive, unsuited for defense let alone hunting; try jumping on the back end of a buffalo and taking a bite with only the equipment nature has provided and you will see what I mean. Humans seem like an unlikely apex predator, yet somehow we managed to rise to the top of the planetary food chain and adapt to every environment on Earth.
Anthropologists will say that it was our ancestors' intelligence that was the key to humanity's success. From 1.9 million to 200,000 years ago, our ancestors tripled their brain size. Our large brains allowed us to develop weapons to augment nature's poor standard equipment. Intelligence also allowed us to develop language to better communicate, and to devise complex hunting strategies other pack hunters could only wish for. And make no mistake, hunting played an essential part in human development.
Despite the prattling by PETA and other radical vegetarian groups, our ancestors were meat eaters. The brain consumes 60% of the energy expended by a resting newborn baby. A resting adult's brain uses 25% of its energy, as opposed to 8% used on average by ape brains. Expanding the human brain demanded a new supply of energy. According to a longstanding body of evidence two things allowed humans to evolve bigger brains: meat and cooking.
About 2 million years ago, soon after the emergence of the genus Homo, our ancestors began to eat more meat, butchering animals with stone tools. The first stone tools appear in Ethiopia about 2.7 million years ago, along with evidence that hominids were using them to butcher scavenged carcasses and extract marrow from bones. Big changes in human anatomy don't appear in the fossil record until more than 1 million years later. According to paleoanthropologist Alan Walker of Pennsylvania State University in State College, a H. erectus skull dating to 1.6 million years ago indicates a brain twice the size of an older australopithecine's. Also about that time, archaeological sites show that H. erectus was moving carcasses to campsites for further butchering and sharing. The traditional explanation is that H. erectus was a better hunter and scavenger and ate more raw meat than its small-brained ancestors.
Harvard University primatologist Richard Wrangham thinks that in addition to going carnivore, our ancestors got cooking. Cooking in effect predigests food, allowing primitive humans to digest the same number of calories with less effort. “Even small differences in diet can have big effects on survival and reproductive success,” he maintains. But a diet of wildebeest tartar and antelope carne cruda alone isn't enough to account for these dramatic changes, says Wrangham. He notes that H. erectus had small teeth—smaller than those of its ancestors. Other carnivores adapted to eating raw meat by increasing tooth size. He argues that whereas earlier ancestors ate raw meat, H. erectus must have been roasting it, with root vegetables on the side or as a fallback when hunters didn't bring home the bacon. “Cooking produces soft, energy-rich foods,” he says (see “Food for Thought”).
A time-line of human events. Credit: J. Newfield/Science.
By the time modern humans arrived in Europe around 40,000 years ago, they were adept at hunting large game and had expanded their palates to dine regularly on small animals and freshwater fish. This according to Michael Richards of the University of British Columbia and Erik Trinkhaus of Washington University, reporting in PNAS (see “Isotopic evidence for the diets of European Neanderthals and early modern humans”). By studying the ratios of carbon and nitrogen isotopes from collagen in bones, they traced the main sources of dietary protein of 27 early Europeans and Neandertals. Richards and Trinkhaus found that the oldest known modern human in Europe—the 35,000-year-old jawbone from Pestera cu Oase cave in Romania—got much of his protein from fish. By 30,000 years ago, other modern humans got as much as 20% of their protein from fish. Compared with us, our ancestors ate “meat: more protein, less refined carbohydrates, and no milk,” according to exercise physiologist Loren Cordain of Colorado State University (see “What's for Dinner? Researchers Seek Our Ancestors' Answers”).
I find it interesting that the two things that made us human—eating meat and the use of fire—are now under attack by those who inhabit the lunatic fringe of rich societies. We are told that eating meat is a sin, atavistic behavior suitable only for savages, and that burning things produces CO2 that will surely bring our world to ruin. It seems like many of the modern world's oh-so-sophisticated thinkers have forgotten where we came from and just how cruel and uncaring nature can be. As comedian Jon Stewart said : “The one thing that brought human kind out the darkness was burning things. We rubbed two sticks together and suddenly we could eat cooked food.”
H. neanderthalensis, an extinct relative.
While interviewing noted global warming alarmist Al Gore, Stewart, the host of Comedy Central's The Daily Show, told the former US Vice-President, “you're fighting not just the industrial revolution age, you're fighting all the the years of human progress.” Stewart, an entertainer who is notably left leaning in his politics, managed to sum the situation up nicely: “It's a much more fundamental shift than I think environmentalists realize. It's the idea that the life that humans have carved out was created through a certain thing of burning things we found.” So light a fire, throw something on the barbie and thank our ancestors for eating meat and learning to cook. Without them we might have joined Homo erctus, Homo neanderthalensis and all the other members of the genus Homo in extinction.
Be safe, enjoy the interglacial and stay skeptical.
Our ancestors knew what was on the menu.