Are you worried about rapid global climate change today? If not, you should be, because rising temperatures, altered rainfall patterns, and the ecological shifts they cause imperil our food supply. Yet, as we have already seen, global climate change has long been a major impetus in human evolution because of its effects on the age-old problem of “what’s for dinner?” It turns out that getting enough food in the face of global climate change also triggered the age of humans.
Getting dinner (or, for that matter, breakfast and lunch) probably does not dominate your list of daily concerns, yet most creatures are almost always hungry and preoccupied with the quest for calories and nutrients. To be sure, animals also need to find mates and avoid being eaten, but the struggle for existence is often a struggle for food, and until recently the vast majority of humans were no exception to this rule. Consider also that acquiring food is even more taxing when your habitat alters dramatically, causing the foods you normally eat to vanish or become less common. As we saw, the challenge of finding enough to eat sparked the first two major transformations in human evolution. As Africa became cooler and drier many millions of years ago, fruit became more scattered and scarce, favoring those ancestors who were better able to forage by standing and walking upright. Additional evolutionary responses were big, thick cheek teeth and large faces well suited to eating foods other than fruit, including tubers, roots, seeds, and nuts. Yet, as important as these transformations were, it is hard to think of Lucy and other australopiths as human. Although bipeds, they retained ape-sized brains, and they didn’t speak, think, or eat like us.
Our bodies and the way we behave evolved to be much more recognizably “human” at the dawn of the Ice Age, a truly pivotal period of change in the earth’s climate that was initiated by continued global cooling between 3 and 2 million years ago. Over this period, the earth’s oceans cooled about 2 degrees Celsius (3.6 degrees Fahrenheit). Two degrees may seem trivial, yet as an average of global ocean temperatures, it represents an enormous quantity of energy. Global cooling involved many back and forth shifts, but by 2.6 million years ago the earth had chilled sufficiently to cause ice caps at the poles to expand. Our ancestors had no idea that gigantic glaciers were forming thousands of miles away, but they certainly experienced cycles of habitat change that were intensified by tumultuous geological activity, especially in eastern Africa. Because of a massive volcanic hotspot, the entire region was pushed upward like a soufflé, and then (like some soufflés) the central portion collapsed, forming the Great Rift Valley. The Rift Valley created an extensive rain shadow, drying out much of eastern Africa. The Rift Valley also harbored many lakes, which to this day continue to fill up and then drain out in cycles. Although eastern Africa’s climate was constantly changing, the overall trend was that forests shrank while woodlands, grasslands, and other more arid, seasonal habitats expanded. By 2 million years ago, the region looked much more like the set of The Lion King than Tarzan.
Imagine being a hungry hominin about 2.5 million years ago, living in a shifting mosaic of grasslands and woodlands, and wondering what to eat. How would you cope as preferred foods, like fruits, became scarcer? One solution, which we saw in the big-faced and humongous-toothed robust australopiths, was to focus even more intensively on increasingly prevalent tough, hard foods like roots, tubers, bulbs, and seeds. These hominins must have spent many hours a day arduously chewing, chewing, and chewing. Fortunately for us, natural selection seems to have also favored a second, revolutionary strategy to cope with changing habitats: hunting and gathering. This innovative way of life involved continuing to gather tubers and other plants but incorporated several new, transformative behaviors that included eating more meat, using tools to extract and process foods, and cooperating intensively to share foods and other tasks.
The evolution of hunting and gathering underlies the evolution of the human genus, Homo. Moreover, the key adaptations that were selected to make this ingenious way of life possible among the first humans were not big brains, but modern-shaped bodies. More than anything else, the evolution of hunting and gathering spurred your body to be the way it is.
The Ice Age precipitated the evolution of hunting and gathering along with modern bodies in several species of early Homo, but the most important is H. erectus. This consequential species has figured prominently in our understanding of human evolution since 1890, when Eugène Dubois, an intrepid Dutch army doctor inspired by Darwin and others, set off to Indonesia to find the true missing link between humans and apes. Blessed with good luck, Dubois found a fossil skullcap and a femur within months of arriving and promptly named it Pithecanthropus erectus (“upright apeman”). Then in 1929, comparable fossils were found in a cave near Beijing (then Peking), China, and named Sinanthropus pekinensis. In the ensuing decades more fossils of a similar nature started turning up in Africa, at Olduvai Gorge in Tanzania, and in places like Morocco and Algeria in North Africa. As with the Peking Man fossils, many of these finds were initially given new species names, and it wasn’t until after World War II that scholars came to the conclusion that the far-flung specimens actually belonged to a single species, H. erectus. According to the best evidence currently available, H. erectus first evolved in Africa by 1.9 million years ago and then rapidly started to disperse from Africa into the rest of the Old World. H. erectus (or a closely related species) shows up in the Caucasus Mountains of Georgia by 1.8 million years ago and in both Indonesia and China by 1.6 million years ago. In parts of Asia, the species persisted until less than a few hundred thousand years ago.
As you might expect for a species that endured for almost 2 million years on three continents, H. erectus came in a variety of shapes, much as we still do. They ranged from 40 to 70 kilograms (88 to 150 pounds) in weight, and from 122 centimeters to more than 185 centimeters (4 feet to almost 6 feet) in height. Many of them were the size of humans today, but females were at the smaller end of the human range, as was an entire population discovered in Georgia (at a site named Dmanisi). If you met a group of H. erectus on the street, you’d probably recognize them as being extremely humanlike, especially from the neck down. Unlike australopiths, their bodies had modern human proportions with relatively long legs and short arms. They had tall, narrow waists and completely modern feet, but their hips flared out more to the side than ours. Like us, they had low, wide shoulders and broad, barrel-shaped chests. But their heads were not entirely like ours. Although H. erectus didn’t have snouts, their faces were tall and deep, and males especially had an enormous, barlike browridge above their eyes. H. erectus brains were intermediate in size between australopith and human brains, and their skulls were long and flat on top and angled out at the back instead of being round like ours. Their teeth were nearly identical to human teeth today, but just a little larger.
~~The Story Of The Human Body : Evolution, Health, and Disease -by- Daniel Lieberman.
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