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Home | Origins | Humans | Evidence | Visions | Timeline Fossils Could Force Rethink of Human Evolution
Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York City, said the new fossils support existing evidence that more than one species of Homo, the genus to which our species belong, inhabited Africa about 1.5 million years ago. "This is definitely evidence for two coexisting lineages," said Tattersall, who was not involved in the study. The discoveries could force scientists to rethink the evolutionary relationship between the two species but some scientists are skeptical. Sister Species H. habilis is the earliest known member of the genus Homo. And H. erectus was the first human ancestor to resemble modern humans. Due to the many overlaps in their anatomy, it was previously thought that H. erectus was descended from H. habilis. While that might still be the case, the new findings open the possibility that the H. habilis and H. erectus once shared a common ancestor from whom they split. The researchers identified one of the fossils as a 1.44 million-year-old partial jawbone belonging to H. habilis. Prior to the discovery, the most recent H. habilis fossil was a 1.6 million-year-old specimen discovered by paleoanthropologist Richard Leakey and his team in 1984 and dubbed "Turkana Boy." The other fossil uncovered at the site is a 1.55 million-year-old skullcap undoubtedly belonging to H. erectus. Its small size compared to other known H. erectus skulls indicates the species might have been sexually dimorphic, with males physically larger than females. Most sexually dimorphic primates, such as gorillas and baboons, tend to mate with multiple partners, so it's possible H. erectus did as well, the researchers say. The two fossils were discovered in 2000 in eons-old volcanic ash in the Illeret region of Kenya, just east of Lake Turkana where Richard Leakey made his discovery. According to standard theory, H. erectus evolved from H. habilis, perhaps by way of an intermediate known as H. ergaster. The relationship between H. erectus and us, Homo sapiens, is murky and controversial. According to the "Out of Africa" human migration model, H. erectus was the first early human to leave Africa in substantial numbers, but it was later replaced by members of H. sapiens, which made the same journey. According to an alternative idea of our species' origin, the scattered bands of H. erectus that originally left Africa simultaneously evolved into H. sapiens in different parts of the world. Possible Link to Lucy's Ancestors Found
Now, Australopithecus fossils found in the Woranso-Mille area of the Afar Region, Ethiopia, fill the date gap between A. anamensis (4.2 to 3.9 million years ago)—and the Lucy species (3.0 to 3.6 million years ago). The species identifications for all the bones remain uncertain, though it appears that some are A. afarensis. Yohannes Haile-Selassie, a physical anthropologist at the Cleveland Museum of Natural History, says his team's 2007 field season in the Woranso-Mille region uncovered the key evidence. "We recovered fossil hominids that date to between 3.5 and 3.8 million years ago," Haile-Selassie said in a prepared statement. "These specimens sample the right time to look into the relationship between Australopithecus anamensis and Australopithecus afarensis and will play a major role in testing the ancestor-descendant hypothesis." The team had found teeth from this time frame at the site over the past few years, but the new material includes more complete jaws that will enable better comparisons, he said. "We have about 35 specimens, mostly isolated teeth, but including one partial skeleton which we believe will give us a lot of information on the post-cranial morphology of early human ancestors," says Haile-Selassie. At least 40 hominid specimens have been recovered from the site so far, including the complete jaws and the partial skeleton. The latter was found in 2005. The team started work in the Woranso-Mille region in 2004 and quickly started finding fossils of early hominids as old as 3.7 million years old, Haile-Selassie said. "We were sure of their importance," he said, "because we knew right from the beginning how old these bones were. We used what is called biostratigraphy to estimate the age of the fossils. Now, we have radiometric dates from volcanic rocks that we sampled from the study area, and now we have a minimum age of 3.5 million years and a maximum of 3.8 million years." Ethiopia, Kenya and Tanzania have yielded many of the earliest hominid and ape fossils that have allowed anthropologists to piece together the history of human evolution. Why We Walk Upright: Beats Being a Chimp
The idea is just one of many scientists have entertained as reasons for why humans walk on two legs. Recent studies have also suggested that, rather than taking millions of years to evolve from a hunched position as is commonly believed, our early ancestors were already capable of standing and walking upright the moment they descended from the trees. Chimps on a Treadmill The researchers trained five chimps to walk both upright and on all fours on a treadmill. The animals wore masks and energy consumption was measured as a function of how much oxygen was consumed during the exercise. The chimp results were compared to four humans walking upright on the treadmills. Results showed that for a given weight, humans used only one-quarter of the energy as the chimps. On average, the chimps used the same amount of energy walking on two legs as they did on four. However, one chimp, with a longer stride, was more efficient walking upright. The team was able to attribute performance differences between the chimps to differences in the length of their stride and the amount of muscle they activated with each step. They measured the latter by having the chimps walk across a metallic force plate. “It’s basically an expensive bathroom scale,” Pontzer explained. “It measures how much force is being applied by the limb to the ground.” Biomechanical equations predict that energy consumption increases either with shorter steps or more active muscle mass. The chimp that used less energy walking bipedally did so because it had longer than average legs. We humans are able to reduce both factors because of our relatively long legs and modified pelvic structures. Reconstructing our Ancestor’s Gait The team has also applied their findings to early hominin fossils. “What [our] results allow us to do was to look at the fossil record and see whether fossil hominins show adaptations that would have reduced bipedal energy expenditures,” said study team member David Raichlen of the University of Arizona. “We and many others have found these adaptations [such as slight increases in hind limb extension or length] in early hominins, which tells us that energetics played a pretty large role in the evolution of bipedalism.” The new findings are in line with the traditional theory of the origins of bipedalism, which states that our ancestors climbed down from the forest canopy some 7 million years ago and began a long transition from walking on all fours to walking upright. However, recent studies have begun to challenge this idea. One team recently found that wild orangutans often stand upright to balance on thin branches. Another found that the muscles gibbons use for climbing and swinging through the forest canopy are also useful for running on two legs, and thus our arboreal ancestors might have been better prepared for walking upright than previously thought. “This would be a different view,” Pontzer told LiveScience. “We don’t need to think about [bipedalism] happening from the tree down. It seems to be very plausible that it would have happened from the ground up.” Clever Apes Recreate an Aesop Fable
All of the orangutans collected water from a drinker and spat it inside the tube to float the peanuts high enough to grab them, averaging three mouthfuls before success. In their first attempts, the apes on average took nine minutes before they got the nuts, but they only needed just 31 seconds by their tenth try. The researchers had to make sure the tube was strong, "because the jaw power of orangutans is enormous," recalled Natacha Mendes, a biologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig. "After so much work constructing tubes, it can be heartbreaking to see it getting destroyed so easily." The findings reminded Mendes of the fable of the thirsty crow, which threw stones into a pitcher to raise and drink the otherwise unreachable water. And the research sheds light on the nature of intelligence among humanity's closest relatives, the great apes, she said. "This is intriguing because it shows they solved problems that go beyond their immediate experience," said Harvard biologist Marc Hauser. In the wild, orangutans are tree-dwellers that don't live near bodies of water, he explained. "It would be interesting to see how flexible they really are, how far they can go beyond what they've evolved to solve." The research was detailed online July 3 in the journal Biology Letters. Chimps More Evolved Than Humans 
The results, detailed in the Proceedings of the National Academy of Sciences, contradict the conventional wisdom that humans are the result of a high degree of genetic selection, evidenced by our relatively large brains, cognitive abilities and bi-pedalism. Jianzhi Zhang of the University of Michigan and his colleagues analyzed strings of DNA from nearly 14,000 protein-coding genes shared by chimps and humans.
They looked for differences gene by gene and whether they caused changes in the generated proteins. Genes act as instructions that organisms use to make proteins and thus are integral to carrying out biological functions, such as transporting oxygen to the body’s cells. Different versions of the same gene are called alleles. Changes in DNA that affect the making of proteins are considered functional changes, while "silent" changes do not affect the proteins. "If we see an excess of functional changes (compared to silent changes) the inference is these functional changes occurred because they were positively selected, because they were useful in some way to the organism," said study team member Margaret Bakewell, also of UM. Bakewell, Zhang and a colleague found that substantially more genes in chimps evolved in ways that were beneficial than was the case with human genes. The results could be due to the fact that over the long term humans have had a smaller effective population size compared with chimps. "Although there are now many more humans than chimps, in the past, human populations were much smaller, and may have been fragmented into even smaller groups," Bakewell told LiveScience. So random events would play a more dominant role than natural selection in humans. Here is why: Under the process of natural selection, gene variants that are beneficial get selected for and become more common in a population over time. But genetic drift, a random process in which chance "decides" which alleles survive, also occurs. In smaller populations, a fortuitous break for one or two alleles can have a disproportionately greater impact on the overall genes of that population compared with a larger one. Chance events could also explain why the scientists found more gene variants that were either neutral and had no functional impact or negative changes that are involved in diseases. There is still much to learn, the scientists say, about human and chimp evolution. "There are possibly a lot of differences between human and chimps that we don’t know about, [perhaps] because there are differences in chimps that nobody has studied; a lot of studies tend to focus on humans," Bakewell said. Evolution Occurs Faster at the Equator  The finding, detailed in the May 2 issue of the journal for the Proceedings of the National Academy of Sciences, could help explain why rainforests have such rich biodiversity compared to other parts of the planet. A census of all the plants and animals around the world would reveal that species richness is uneven: it is highest in the tropics, the regions of Earth near the equator, and lower the closer one goes toward the planet's poles. To investigate the reasons for this trend, Shane Wright of the University of Auckland, New Zealand, and colleagues looked at the rate of molecular evolution for 45 tropical plants and compared it to that of related species living at more temperate latitudes. The researchers examined the rate at which DNA bases in the plants' genetic code are substituted. Like characters in a four-letter alphabet, bases are DNA molecules arranged to spell out instructions for building proteins. If one of the letters—A, T, G or C—become substituted with another, the instructions can change and a dysfunctional or entirely new and useful protein can be produced. The researchers found that tropical plants had more than twice the rate of base substitution compared to their temperate cousins. The finding supports a theory put forth by biologist Klaus Rohde in 1992 that climate can have a powerful effect on how fast organisms evolve and branch off into new species. Scientists think it works like this: Warmer temperatures speed up metabolism by allowing chemical reactions to occur at a faster rate, but this increased efficiency comes at a price: it produces higher quantities of charged atoms or molecules called "free radicals," which can damage biological molecules like proteins and so-called "nucleic acids" such as DNA. Higher metabolism also speeds up DNA replication, which is just another chemical reaction, and this can increase the number of copying mistakes that can occur.
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