Sunday, March 14, 2010
how neanderthals disappeared
Coyne writes that "what really happened to [neanderthals] is arguably the biggest unknown about human evolution" (206). The two main theories are that neanderthals simply evolved into other species in all the areas it had inhabited, or that homo sapiens had migrated out of Africa and out-competed neanderthals into extinction world-wide. Based on the traces that evolution leaves and speciation in geographically separated areas, which theory on the extinction of neanderthals is more likely to have occurred? Could it have been a combination of both theories?
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The book explains that there are two theories to what happened in the evolution of Homo sapiens. The first is the multiregional theory. This theory proposes that there was an evolutionary replacement and H. erectus simply evolved into H. sapiens independently in several areas because natural selection was acting in the same way all over Asia, Europe, and Africa. The second theory is the "out of Africa" theory, which proposes that modern H. sapiens originated in Africa and spread, physically replacing the H. erectus and the Neanderthals by outcompeting them for food or killing them. It seems that the second theory is more supported because there is more genetic and fossil evidence for this theory. The multiregional theory suggests that there is fifteen times more genetic difference between races due to the splitting of populations over a million years ago as opposed to our ancestors leaving Africa only 60,000 years ago in the "out of Africa" theory. Both theories, however, must consider the main facts; "Humans first left Africa and established populations in other parts of the world (first southern Asia, China, and Java, later Europe) by 1.8 million years ago, humans today are quite different anatomically and behaviorally from archaic people (that is, most humans before 40,000 years ago) anywhere in the world, recent people are called "modern" humans, human populations today are genetically very similar to each other, African populations today are more genetically diverse than populations in other parts of the world, and recent humans in Europe and Asia share a few features with the ancient archaic people who lived in those places before 40,000 years ago." ( http://johnhawks.net/weblog/topics/modern_human_origins/multiregional_vs_out_of_africa.html) Both theories address each issue differently, which makes it difficult to determine which theory is more correct. It was discovered in 2003, on the island of Flores in Indonesia, the fossil of Homo floresiensis. They were one meter tall and weighted fifty pounds. Their brains were also very small but their teeth and skeletons were indisputably those of Homo. They used stone tools and many preyed on Komodo dragons and dwarf elephants that populated the island. The fossils date to 18,000 years ago, well after Neanderthals disappeared and twenty-five centuries after modern H. sapiens had reached Australia. Scientists have guessed that they florensiensis represents an isolated population of H. erectus that colonized Flores and was somehow bypassed by the spread of modern H. sapiens. Some argue that the fossils of the tiny sized skull may simply represent a diseased individual of modern H. sapiens with hypothyroid cretinism, which is a condition producing abnormally small skulls and brains; however, the recent analysis of the fossils wrist bones support that H. floresiensis was a genuine species of hominin. Either way, when looking in chronological order at the fossils presented as evidence, fossils that started off ape like became more and more similar to modern humans as time went on. It is clear that our divergence from the ancestors of chimps occurred in East or Central Africa about 7 million years ago, and that bipedal walking evolved well before the evolution of large brains.
ReplyDeleteI believe that the neanderthal extinction is probably due to the second theory, "out of Africa" because there is more evidence genetically, anatomically, and archeologically supporting this theory. Anatomically, neanderthals had "an oddly shaped occipital region of the skull with a bulge or bun, molars with enlarged pulp chambers, and large, often very heavily worn incisors, a mandible lacking a chin and possessing a large gap behind the last molar, a massive thorax, short forearms and lower legs, robustly built skeletons with thick walled limb bones, and long clavicles and very wide scapulas." Homo sapiens, however, had "a cranial vault with a vertical forehead, rounded occipital and reduced brow ridge, reduced facial skeleton lacking a projecting mid-face, lower jaw sporting a chin, more modern, and less robustly built skeleton." This suggests thatNeanderthals and early modern humans had been isolated from one another and were evolving separately into two distinct species. Archeologically, "shortly after fully modern humans entered Europe, roughly 40,000 years ago, the Neanderthals began a fairly rapid decline, culminating in their disappearance roughly 30,000 years ago. Neanderthals were apparently no match for the technologically advanced fully modern humans who invaded Europe and evidence for interbreeding of these two types of hominids is equivocal." Lastly genetically, "studies done have support the view that Neanderthals did not interbreed with Homo sapiens who migrated into Europe. It is, therefore, highly likely that modern humans do not carry Neanderthal genes in their DNA." (http://www.actionbioscience.org/evolution/johanson.html)
ReplyDeleteI think it is also important to look at how humans may have evolved in order to determine which theory holds the most truth. The complex adaptations of humans, such as, the large brains, erect posture, and smaller teeth are hard to explain when looking at an evolutionary perspective. The climate between 10 million and 3 million years ago, however, was change drastically in East and Central Africa due to a drought. The climate became dryer due to the erratic periods of drought and rainfall. The dry periods gave way to more open habitats such as savannas, grasslands, open forests, and desert scrub. Biologists believe the open habitats gave way to the bipedal walking, which was the first significant hominin trait to evolve. This trait may have evolved to allow individuals to collect foods with their free hands, help individuals deal with high temperatures by raising bodies off the ground, and reducing surface area exposed to sun. Once individuals hands were free, they could potentially fasten small tools, which could have lead to the developing of a larger brain size. It may not be clear exactly how these traits evolved, whether or not we evolved from neanderthals and H. erectus or that humans evolved separately and outcompeted these organisms; however, it is clear that humans evolved due to the evidence of comparative anatomy, embryology, vestigial organs, dead genes, transitory fetal coat of hair, our poor design, and the fossil record.
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