Human evolution_su_tien_hoa_loai_nguoi Human evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoiHuman evolution_su_tien_hoa_loai_nguoi
Human Evolution —By Karen Barss The story of human evolution began in Africa about six million years ago and it describes the very long process that our ancestors went through to ultimately become modern humans This process has been uncovered by studying fossils and understanding the underlying theory of evolution, and while new fossils are uncovered every decade revealing new chapters, scientists agree about the basic story What Is Evolution? Evolution means the changes that occur in a population over time In this definition, a “population” means a group of the same species that share a specific location and habitat Evolutionary changes always occur on the genetic level In other words, evolution is a process that results in changes that are passed on or inherited from generation to generation It does not, for example, describe how people can change their muscle mass by lifting weights When successful, these genetic changes or adaptations, which happen when genes mutate and/or combine in different ways during reproduction, help organisms survive, reproduce, and raise offspring Some individuals inherit characteristics that make them more successful at surviving and having babies These advantageous characteristics tend to appear more frequently in the population (because those individuals with less advantageous characteristics are more likely to die without reproducing), and over time these changes become common throughout that population, ultimately leading to new species The Tree of Life Biological evolution explains the way all living things evolved over billions of years from a single common ancestor This concept is often illustrated by the socalled tree of life Every branch on the tree represents a species The fork separating one species from another represents the common ancestor that each pair of species shared So ultimately, all life is interconnected, but any two species may be separated by millions or even billions of years of evolution Only a Theory? Some people dismiss evolution as “just a theory.” Evolution is in fact a theory, a scientific theory In everyday use, the word theory often means a guess or a rough idea: “My theory is…” “I have a theory about that.” But among scientists, the word has an entirely different meaning In science, a theory is an overarching explanation used to describe some aspect of the natural world that is supported by overwhelming evidence Other scientific theories include cell theory, which says that all living things are made up of cells, and heliocentric theory, which says the earth revolves around the sun instead of the other way around The Relationship between Apes and Humans Since scientists developed the ability to decode the genome and compare the genetic makeup of species, some people have been stunned to learn that about 98.5% of the genes in people and chimpanzees are identical This finding means chimps are the closest living biological relatives to humans, but it does not mean that humans evolved from chimps What it does indicate is that humans share a common ancestor with modern African apes (i.e., gorillas and chimpanzees), making us very, very distant cousins We are therefore related to these other living primates, but we did not descend from them Modern humans differ from apes in many significant ways Human brains are larger and more complex; people have elaborate forms of communication and culture; and people habitually walk upright, can manipulate very small objects, and can speak Our Common Ancestor Most scientists believe our common ancestor existed to million years ago Then two species broke off into separate lineages, one ultimately evolving into gorillas and chimps, the other evolving into early humans called hominids In the millions of years that followed, at least a dozen different species of humanlike creatures have existed, reflected in the fossil discoveries of paleoanthropologists, although many of these species are close relatives but not actual ancestors of modern humans In fact, the fossil record does not represent a straight line of ancestry at all; many of these early hominids left no descendents and simply died out Still others are most likely direct ancestors of modern humans or Homo sapiens While scientists still not know the total number of hominid species that existed, because new fossils are discovered every decade, the story of human evolution becomes clearer all the time What about the Missing Link? The idea of a missing link has persisted, but it is not actually a scientific term In the popular imagination, this missing link would be the fossil of our common ancestor While scientists agree on the concept of a common ancestor, deciding which fossil represents that actual species is challenging if not impossible, given that the fossil record will never be 100% complete Also, the word implies that evolution is a straight chain of events, when in fact the sequence of evolution is much more complicated The Fossil Record Fossils are the remains or impressions of living things hardened in rock All living organisms have not been preserved in the fossil record; in fact, most have not because very specific conditions must exist in order to create fossils Even so, the fossil record provides a fairly good outline of human evolutionary history The earliest humans were found in Africa, which is where much of human evolution occurred The fossils of these early hominids, which lived to million years ago, all come from that continent Most scientists believe early humans migrated out of Africa into Asia between million and 1.7 million years ago, entering Europe some time within the past million years What follows are some highlights of the early human species that have been identified by scientists to date Australopithecines An African apelike species evolved probably around million years ago with two skeletal characteristics that set it apart from apes: small canine teeth (the teeth on either side of the four front teeth) compared to the long canines found in almost all other primates, and, most importantly, bipedalism or walking on two legs as the primary mode of locomotion The name australopithecine means “southern ape,” in reference to South Africa where the first known fossils were found Many more australopith fossils have been found in the Great Rift Valley in eastern Africa, in countries including Ethiopia, Tanzania, Kenya, and Chad The very early years of the transition from ape to human, from million to million years ago, is poorly documented in the fossil record, but those fossils that have been discovered document the most primitive combinations of ape and human features Fossils from different early australopith species that lived between million and million years ago show a variety of adaptations that mark this transition much more clearly Among the genera that are included in early australopith species are Sahelanthropus, Orrorin, and Aripithecus; a species of the genus Kenyanthropus; and four species of the genus Australopithecus Probably the best-known australopith specimen is “Lucy,” the partial skeleton of a female discovered in 1974 in Hadar, Ethiopia Lucy belongs to a species, Australopithicus afarensis, which thrived in eastern Africa between 3.9 million and million years ago Scientists have found several hundred A afarensis fossils in Hadar Lucy lived 3.2 million years ago Another very exciting A afarensis site was discovered in northern Tanzania at Laetoli In addition to fossilized bones of A afarensis, researchers in 1978 discovered trails of bipedal human footprints preserved in hardened volcanic ash over million years ago The footprints provided irrefutable evidence that australopiths regularly walked upright By about 2.7 million years ago, so-called robust australopiths (in contrast to the earlier, gracile forms) had evolved, with wide molars and premolars and a facial structure that indicate that these robust australopiths chewed their food, primarily tough, fibrous plants, powerfully and for long periods Several robust species have been identified, and the last robust australopiths died out about 1.4 million years ago The Genus Homo The genus Homo first evolved at least 2.3 million to 2.5 million years ago The most significant difference between members of this genus and australopiths, with which they overlapped, was their significantly larger brains (about 30 percent larger, though still small compared to modern humans) Scientists divide the evolution of the modern human genus into three rough periods: early, middle, and late Species of early Homo, among them Homo habilis, resembled australopiths in many distinct ways, but they had smaller teeth and jaws, more modern-looking feet, and hands capable of making tools They probably lived from between 2.5 or 2.3 million and 1.6 million years ago The middle Homo species, including Homo erectus, evolved anatomically to be more similar to modern humans but their brains were relatively small (though bigger than australopiths) They probably overlapped with earlier Homospecies, as they developed perhaps between million and 1.8 million years ago Homo erectus was a very successful species of the middle period; fossils have been found throughout Africa, Europe, and much of Asia, and the species may have survived for more than 1.5 million years The final transition, from the middle to late periods, happened about 200,000 years ago Late Homo species, including Neanderthals and Homo sapiens, evolved large and complex brains, leading eventually to language, and developed culture as an increasingly important aspect of human life Homo sapiens Scientists have dated the oldest known fossils with skeletal features typical of modern humans from 195,000 years ago Early anatomically modern Homo sapiens fossils have come from sites in Sudan, Ethiopia, South Africa, and Israel Many scientists have therefore concluded that modern Homo sapiens evolved in Africa and began spreading to other parts of the world 90,000 years ago or a little earlier, although whether, how, why, and when this happened is still in dispute And it was not until about 40,000 years ago that anatomically modern humans, Homo sapiens sapiens, emerged Since that time, human evolution has been primarily cultural as opposed to biological Putting Human Evolution in Perspective Humans have existed for only a tiny fraction of Earth’s history Scientists believe Earth itself is approximately 4.55 billion years old The oldest known fossils are about 3.5 billion years old, although some scientists have discovered evidence that life may have begun nearly billion years ago Dinosaurs walked Earth between 230 and 65 million years ago The oldest known humanlike fossil has been dated at 4.4 million years old, although another species, not yet confirmed as a hominid, has been dated at about million years old As mentioned earlier, scientists estimate that the earliest hominid species diverged from the ape lineage between and million years ago And yet, the species to which we belong, Homo sapiens sapiens, is only about 40,000 years old New fossil findings demonstrate that the diminutive hominin Homo floresiensis lived on the Indonesian island of Flores at least 700,000 years ago, and may point to its rapid dwarfism from the larger Homo erectus See Letters p.245 & p.249 Since the first description of Homo floresiensis in 2004 (ref 1), these little hominins from the Indonesian island of Flores have raised very big questions Do these skeletal remains represent a new species in the extinct hominin family, or are they modern humans who were pathologically dwarfed, or members of a short-stature population? If they belong to a different species, what was its evolutionary origin? Why was it so different from other hominin species? The most common answer to these questions has been repeated for more than ten years: we need more remains from Flores — especially from different sites and older time periods — to tip the scales On pages 245 and 249 of this issue, van den Bergh et al.2 and Brumm et al.3 report the finding of those long-awaited remains After H floresiensis was described, many palaeoanthropologists embraced the idea of a new and odd-looking hominin species that had a diminutive brain and body size Supporters of the pathology hypothesis, however, have been unrelenting in looking for syndromes and conditions that could have been responsible for the unexpectedly small size of these hominins; some suggestions have been published quite recently4 The current findings — consisting of a lower-jaw fragment, an indeterminate cranial fragment and some small teeth from at least three different individuals — confirm beyond any reasonable doubt that H floresiensis is a distinct hominin species with deep evolutionary roots that trace back more than 700,000 years Van den Bergh and colleagues present detailed analyses of the size and shape of the fossils, found at the Mata Menge site on Flores, including comparisons with remains of other hominin species They show that such tiny teeth are found only in Homo sapiens — whose origin and migration to Asia are substantially later than the age of the new fossils — and in H floresiensis Brumm and colleagues report on the open-grassland habitat and stone tools associated with these hominins They describe these tools as technologically similar to the ones found with the later H floresiensis individuals from the Liang Bua site, and suggest that this points to the behavioural stability of the hominins from Flores over a long period of time In addition, Brumm et al use a combination of dating techniques to provide evidence that the fossils were deposited around 700,000 years ago, thus confirming the early origin of this species Although this confirmation finally ends the debate about the validity of H floresiensis as a species, its evolutionary origins are likely to remain under discussion for much longer There are two main models (Fig 1) H floresiensis may have evolved from the larger Homo erectus through a process of island dwarfing — an extreme reduction in size due to the absence of predators and to resource scarcity that is typical of island ecosystems Alternatively, it may be descended from the earlier Homo habilis, or even from a small form of hominin from the Australopithecusgenus This second model implies that very primitive hominins would have left Africa by million years ago, but there is no fossil or archaeological evidence for such an early dispersal Figure 1: Candidates for the ancestry of Homo floresiensis There are two main models for the evolutionary origin of the hominin species H floresiensis, which inhabited the Indonesian island of Flores and had a particularly small brain and body One possibility is that Homo habilis, or a similar form that also had a relatively small body and brain, may have left Africa by million years ago and reduced in size even further But there is no evidence for such early hominins outside Africa Alternatively, H floresiensis may be descended from the later and larger-bodied Homo erectus, for which there is evidence on Java around million years ago and earlier This second model would involve much greater body and brain reduction over a much shorter period of time (Data on brain and body size are from refs 12–14, and are based on east African specimens for H habilis, on early Indonesian specimens for H erectus and on remains from Liang Bua for H floresiensis.) Full size image Mostly on the basis of the morphology of a lower molar tooth and of general affinities of the jaw fragment, van den Bergh and colleagues claim that the remains from Mata Menge are more closely related to H erectus than to H habilis The reliability of lower-molar morphology to assess species relationships supports their claim5 However, the traits that point to a more primitive ancestor for H floresiensis mostly come from body parts other than the skull6,7 and cannot be assessed using the Mata Menge sample, which does not include such postcranial remains Without further fossil evidence, the discussion between proponents of the two models will continue Some will think that extreme dwarfing from H erectus is unlikely, especially to the extent of the dramatic brain-size reduction observed in H floresiensis8, although empirical data from hippopotamuses suggest that similarly strong brain reduction may occur9 Others will argue that a longdistance migration route for H habilis, or an earlier form, from Africa to southeast Asia is even more implausible For now, it seems that all possible explanations remain outside the comfort zone of classic scenarios of human evolution Van den Bergh et al propose that the hominins from Mata Menge might be descended from the hominins that made stone tools at the site of Wolo Sege10, also on Flores, which is dated to approximately million years ago They further hypothesize that large-bodied H erectushominins are the ones that made these tools This speculation could be proved wrong if remains from other small hominins were found with the tools in the future, but it raises an interesting question: can the extreme reduction of the brain and body of H floresiensis have evolved over a mere 300,000 years? Three hundred millennia may not seem a 'short' period of time to many readers However, no other such dramatic transformation in hominin evolution is known to have occurred over a similarly brief timescale A quantitative analysis and comparison of evolutionary rates across different hominin species and with H floresiensis would lend formal support to this informal observation Alongside such quantification, it might be helpful to look at more-distant species Some mammals show evidence of even stronger degrees of dwarfing over substantially shorter periods of time, and extremely fast rates of size reduction in island environments11 In addition, we must not rule out the possibility that the direct ancestors of H floresiensis were not the most typical representatives of their species Indeed, the strange combination of primitive and derived traits in H floresiensis anatomy could be the result of a pronounced founder effect, which occurs when a new population is established from a small sample that does not reflect the parental population's diversity and most-common traits Some scenarios that look mind-blowing become underwhelmingly conventional island dwarfism is not extraordinary in long-scale migration of species that from our anthropocentric point of view when we expand our horizons Rapid nature, nor is the founder effect or the lack human-like cognitive abilities Whatever the actual origin of H floresiensis, we will be much closer to an answer if we look beyond hominins in our search for explanations ... anatomically modern humans, Homo sapiens sapiens, emerged Since that time, human evolution has been primarily cultural as opposed to biological Putting Human Evolution in Perspective Humans have existed... the fossil record provides a fairly good outline of human evolutionary history The earliest humans were found in Africa, which is where much of human evolution occurred The fossils of these early... chimps are the closest living biological relatives to humans, but it does not mean that humans evolved from chimps What it does indicate is that humans share a common ancestor with modern African