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Haile-Selassie and his colleagues haven't collected enough bones yet to reconstruct with great precision what kadabba looked like. But they do know it was about the size of modern common chimpanzees, which when standing average about 4 ft. tall. That makes it roughly the same size as its close relative A. ramidus ramidus and about 20% taller than Lucy, the famous 3.2 million-year-old human ancestor discovered about 50 miles away in 1974 that is even further along the evolutionary track. The size of kadabba's brain and the relative proportions of its arms and legs were probably chimplike as well.
But unlike a chimp or any of the other modern apes that amble along on four limbs, kadabba almost certainly walked upright much of the time. The inch-long toe bone makes that clear. Two-legged primates (modern humans included) propel themselves forward by leaving the front part of their foot on the ground and lifting the heel. This movement, referred to as toeing off, causes the bones in the middle of the foot to take on a distinctive shape--a shape that is readily apparent in the ancient toe bone. "If you compare a chimp's foot bones with its hand bones, they look the same because they're used for the same thing"--that is, for grasping--Haile-Selassie explains. "Hominid fingers and toes don't look alike at all."
Exactly how this hominid walked is still something of a mystery, though with a different skeletal structure, its gait would have been unlike ours. Details of kadabba's lifestyle remain speculative too, but many of its behaviors undoubtedly resembled those of chimpanzees today. It probably still spent some time in trees. It probably lived in large social groups that would include both sexes. And rather than competing with one another for mates, the males may well have banded together to defend the troop against predators, forage for food and even hunt for game.
But that kadabba walked upright at all is hugely significant. Paleontologists have suspected for nearly 200 years that bipedalism was probably the key evolutionary transition that split the human line off from the apes, and fossil discoveries as far back as Java Man in the 1890s supported that notion. The astonishingly complete skeleton of Lucy, with its clearly apelike skull but upright posture, cemented the idea a quarter-century ago.
What's been much tougher to pin down is just why two-leggedness arose. The conventional wisdom has long focused on the fact that eastern Africa became significantly dryer about the time that humans first evolved. The change would have tended to favor grasslands over forests, and, so went the theory, our ancestors changed to take advantage of the new conditions. We learned to walk upright so that we could see over the tall grasses to spot predators coming; an upright posture, moreover, would offer a much smaller target for the oppressive heat of the grassland sun, and a larger target for cooling breezes.