人類演化簡史 -- C. Q. Choi
Human Evolution: Where We Came From
Charles Q. Choi, Special to LiveScience, livescience.com, 11/02/09
Editor's Note: This is Part 4 in 10-part series LiveScience series on the origin, evolution and future of the human species and the mysteries that remain to be solved.
The dawn of humanity remains a fascinating mystery. What started our distant ancestors on the evolutionary path that led to us?
Spectacular fossils and a host of other data uncovered in the last decade are revealing key details to solving this riddle. As often than not, however, these clues raise as many questions as they answer.
Ardipithecus - the first hominid?
The latest fossil relative of humanity to be unearthed - Ardipithecus ramidus - is among the most extraordinarily detailed. It may also be potentially crucial to our understanding of human evolution.
A remarkably complete female of the species dubbed "Ardi" and at least 35 members of her species were discovered in Ethiopia and are roughly 4.4 million years old. This date is significant, as genetic evidence suggests our branch of the primate family tree - often known as the hominids - appeared roughly 4 million years ago after diverging from the last common ancestor we shared with chimps.
Ardipithecus "is the first creature on our side of the family tree," said paleoanthropologist Tim White of the University of California at Berkeley, a leader of the team that discovered Ardi.
The holy grail of the study of human evolution would be discovering the first hominid, as it would shed light on what first drove our ancestors to develop the way they did. Darwin conjectured tool use was the key, driving us to bipedalism to better use our hands, but fossil evidence shows bipedalism came millions of years before larger brains or stone tools did, and chimpanzees, our closest living relatives, clearly know how to make and use tools while staying as a quadruped, on all fours.
For a long time now, scientists have instead suggested that bipedalism - an upright posture on two legs - was the key adaptation that set us on the line to becoming human, "but I think that is actually much less clear now with Ardi," said paleoanthropologist John Hawks at the University of Wisconsin at Madison.
While Ardi was better at walking upright than chimpanzees are, it still would have been poor at it and must have spent a lot of time in the trees, possessing as she did an opposable big toe and other adaptations of the foot and pelvis best developed for climbing. Indeed, "her generalized pelvis might be the default form, with the long pelvis of chimps and gorillas, adapted for having their trunks in a horizontal position, being derived from that," Hawks said.
What makes Ardipithecus a clear relative of humanity are its small, human-like canine teeth.
Virtually all male primates except hominids have large canines - in chimpanzees and gorillas, these are tusk-like, and when the upper canines close down, they are sharpened against lower teeth, keeping them honed as weapons for threatening and sometimes attacking other males.
This evolutionary change in canines might explain why bipedalism evolved in the first place, suggested anatomist C. Owen Lovejoy at Kent State University, who collaborated with White on Ardi. Since the canines were no longer used as weapons, Lovejoy conjectured this reflected a change in behavior, involving less competition between males. To entice females, males instead might have brought them food, and bipedalism would free up the arms to carry such gifts.
Still, Hawks noted that earlier fossils, such as the roughly 7-million-year-old Sahelanthropus and 6-million-year-old Orrorin, also displayed similar teeth, which meant reduction in canine size might have occurred long before bipedalism, sometimes called bipedality.
"We see now with Ardi that bipedality is part of a broader change, but I think we have yet to identify what that is - dietary or ecological or even social factors," Hawks said.
Australopithecus - the ancestor of humanity?
While Ardipithecus may represent the first hominid, it may be Australopithecus that may be the ancestors of Homo, the group that we humans, Homo sapiens, belong to. Although Ardi had an intermediate form of walking, the famed 3.2-million-year-old skeleton Lucy, which belonged to the species Australopithecus afarensis, definitely strode upright.
The australopithecines lived from about 4.2 million years ago to roughly 2 million years ago, living in open woodlands. "Most of us think Homo came from some version of Australopithecus," Hawks said. One major candidate is Australopithecus garhi, which White helped discover in 1996.
Homo emerged about 2.5 million years ago, coinciding with the first evidence of stone tools. "The environment got drier about then. There was drying throughout Africa, and spreading of grassland species," Hawks said. "Horses enter about 2.3 million years ago, the ancestors of the zebra, and you had grassland antelopes diversifying."
It is possible that Ardipithecus is a hominid, and that Australopithecus is its direct descendent. It is also possible that Ardipithecus is the common ancestor of chimpanzees and our ancestors, and that Australopithecus is the first hominid, Hawks said, or that Ardipithecus is a side branch of our family tree.
"Everyone wants to find the first," Hawks said.
本文於 修改第 2 次
現代歐洲人的祖先 - P. Rincon
Europeans drawn from three ancient 'tribes'
Paul Rincon, 09/17/14
The modern European gene pool was formed when three ancient populations mixed within the last 7,000 years, Nature journal reports.
Blue-eyed, swarthy hunters mingled with brown-eyed, pale skinned farmers as the latter swept into Europe from the Near East.
But another, mysterious population with Siberian affinities also contributed to the genetic landscape of the continent.
The findings are based on analysis of genomes from nine ancient Europeans.
Agriculture originated in the Near East - in modern Syria, Iraq and Israel - before expanding into Europe around 7,500 years ago.
Multiple lines of evidence suggested this new way of life was spread not just via the exchange of ideas, but by a wave of migrants, who interbred with the indigenous European hunter-gatherers they encountered on the way.
But assumptions about European origins were based largely on the genetic patterns of living people. The science of analysing genomic DNA from ancient bones has put some of the prevailing theories to the test, throwing up a few surprises.
Genomic DNA contains the biochemical instructions for building a human, and resides within the nuclei of our cells.
In the new paper, Prof David Reich from the Harvard Medical School and colleagues studied the genomes of seven hunter-gatherers from Scandinavia, one hunter whose remains were found in a cave in Luxembourg and an early farmer from Stuttgart, Germany.
The hunters arrived in Europe thousands of years before the advent of agriculture, hunkered down in southern refuges during the Ice Age and then expanded during a period called the Mesolithic, after the ice sheets had retreated from central and northern Europe.
They were closely related to each other and their genetic profile is not a good match for any modern group of people, suggesting they were caught up in the farming wave of advance. However, their genes live on in modern Europeans, to a greater extent in the north-east than in the south.
The early farmer genome showed a completely different pattern, however. Her genetic profile was a good match for modern people in Sardinia, and was rather different from the indigenous hunters.
The Sardinians could represent a population of early farmers that became isolated on the Mediterranean island, and were little affected by later migrations that shaped the rest of Europe.
But, puzzlingly, while the early farmers share genetic similarities with Near Eastern people at a global level, they are significantly different in other ways. Prof Reich suggests that more recent migrations in the farmers' "homeland" may have diluted their genetic signal in that region today.
Prof Reich explained: "The only way we'll be able to prove this is by getting ancient DNA samples along the potential trail from the Near East to Europe... and seeing if they genetically match these predictions or if they're different.
"Maybe they're different - that would be extremely interesting."
The agricultural transition was a period of momentous cultural and demographic change
These regions will be the most challenging to get ancient samples from, however, because DNA breaks down more readily in warmer climates.
Pigmentation genes carried by the hunters and farmers showed that, while the dark hair, brown eyes and pale skin of the early farmer would look familiar to us, the hunter-gatherers would stand out if we saw them on a street today.
"It really does look like the indigenous West European hunter gatherers had this striking combination of dark skin and blue eyes that doesn't exist any more," Prof Reich told BBC News.
Dr Carles Lalueza-Fox, from the Institute of Evolutionary Biology (CSIC - UPF) in Barcelona, Spain, who was not involved with the research, told BBC News: "If you look at all the reconstructions of Mesolithic people on the internet, they are always depicted as fair skinned. And the farmers are sometimes depicted as dark-skinned newcomers to Europe. This shows the opposite."
Last year, Dr Lalueza-Fox published genetic details of a 7,000-year-old hunter from Spain who was similarly dark and blue-eyed, suggesting these looks were not a one-off.
So where did fair pigmentation in present-day Europeans, including their rich diversity of hair colour, come from? The farmer seems to be on her way there, carrying a gene variant for light skin that's still around today.
"There's an evolutionary argument about this - that light skin in Europe is biologically advantageous for people who farm, because you need to make vitamin D," said David Reich.
"Hunters and gatherers get vitamin D through their food - because animals have a lot of it. But once you're farming, you don't get a lot of it, and once you switch to agriculture, there's strong natural selection to lighten your skin so that when it's hit by sunlight you can synthesise vitamin D."
This reconstruction shows the dark skin and blue eyes of a 7,000-year-old hunter from northern Spain
When the researchers looked at DNA from 2,345 present day people, they found that a third population was needed to capture the genetic complexity of modern Europeans.
This additional "tribe" is the most enigmatic and, surprisingly, is related to Native Americans.
Hints of this group surfaced in an analysis of European genomes two years ago. Dubbed Ancient North Eurasians, this group remained a "ghost population" until 2013, when scientists published the genome of a 24,000-year-old boy buried near Lake Baikal in Siberia.
This individual had genetic similarities with both Europeans and indigenous Americans, but lacked the East Asian ancestry present in Siberia and the Americas today. The ghost had been sighted.
The 8,000-year-old Scandinavian hunters already show some signs of mixture with this population, but the ancient hunter from Luxembourg and the farmer from Germany do not, implying that this third ancestor was added to the continental mix after farming was already established in Europe.
Dr Lalueza-Fox commented: "The interesting point is the idea that we can dissect these components in any modern European and explain diversity in modern Europeans as different proportions of these three populations."
The study also revealed that the early farmers and their European descendents can trace a large part of their ancestry to a previously unknown, even older lineage called Basal Eurasians. This group represents the earliest known population divergence among the humans who left Africa 60,000 years ago.
Follow Paul on Twitter.
More on This Story
, SCIENCE & ENVIRONMENT
, SCIENCE & ENVIRONMENT
, SCIENCE & ENVIRONMENT
, SCIENCE & ENVIRONMENT
本文於 修改第 1 次
人類利他行為的根源與影響 -- The Univ. of Zurich
The roots of human altruism
Apes hardly ever act selflessly without being solicited by others; humans often do. What has caused this curious divergence, which is arguably the secret to our species’ unparalleled success? A team headed by an anthropologist from the University of Zurich now reveals that cooperative care for the young was the evolutionary precondition for the emergence of spontaneous altruistic behavior.
The University of Zurich, 08/27/14
Scientists have long been searching for the factor that determines why humans often behave so selflessly. It was known that humans share this tendency with species of small Latin American primates of the family Callitrichidae (tamarins and marmosets), leading some to suggest that cooperative care for the young, which is ubiquitous in this family, was responsible for spontaneous helping behavior. But it was not so clear what other primate species do in this regard, because most studies were not comparable.
A group of researchers from Switzerland, Germany, Austria, Italy and Great Britain, headed by anthropologist Judith Burkart from the University of Zurich, therefore developed a novel approach they systematically applied to a great number of primate species. The results of the study have now been published in Nature Communications.
For their study, Burkart and her colleagues developed the new paradigm of group service, which examines spontaneous helping behavior in a standardized way. With the aid of a simple test apparatus, the researchers studied whether individuals from a particular primate species were prepared to provide other group members with a treat, even if this meant missing out themselves (see box). The scientists applied this standardized test to 24 social groups of 15 different primate species. They also examined whether and how kindergarten children aged between four and seven acted altruistically.
The researchers found that the willingness to provision others varies greatly from one primate species to the next. But there was a clear pattern, as summarized by Burkart:
“Humans and callitrichid monkeys acted highly altruistically and almost always produced the treats for the other group members. Chimpanzees, one of our closest relatives, however, only did so sporadically.”
Similarly, most other primate species, including capuchins and macaques, only rarely pulled the lever to give another group member food, if at all – even though they have considerable cognitive skills.
Until now, many researchers assumed that spontaneous altruistic behavior in primates could be attributed to factors they would share with humans: advanced cognitive skills, large brains, high social tolerance, collective foraging or the presence of pair bonds or other strong social bonds. As Burkart’s new data now reveal, however, none of these factors reliably predicts whether a primate species will be spontaneously altruistic or not. Instead, another factor that sets us humans apart from the great apes appears to be responsible. Says Burkart:
“Spontaneous, altruistic behavior is exclusively found among species where the young are not only cared for by the mother, but also other group members such as siblings, fathers, grandmothers, aunts and uncles.”
This behavior is referred to technically as the “cooperative breeding” or “allomaternal care.”
The significance of this study goes beyond identifying the roots of our altruism. Cooperative behavior also favored the evolution of our exceptional cognitive abilities. During development, human children gradually construct their cognitive skills based on extensive selfless social inputs from caring parents and other helpers, and the researchers believe that it is this new mode of caring that also put our ancestors on the road to our cognitive excellence. This study may, therefore, have just identified the foundation for the process that made us human. As Burkart suggests:
“When our hominin ancestors began to raise their offspring cooperatively, they laid the foundation for both our altruism and our exceptional cognition.”
J. M. Burkart, O. Allon, F. Amici, C. Fichtel, C. Finkenwirth, A. Heschl, J. Huber, K. Isler, Z. K. Kosonen, E. Martins, E. Meulman, R. Richiger, K. Rueth, B. Spillmann, S. Wiesendanger & C. P. van Schaik (2014). The evolutionary origin of human hyper-cooperation. Nature Communications 5:4747 doi: 10.1038/ncomms5747
Test set-up for the altruism study
A treat is placed on a moving board outside the cage and out of the animal’s reach. With the aid of a handle, an animal can pull the board closer and bring the food within reach. However, the handle attached to the board is so far from the food that the individual operating it cannot grab the food itself. Moreover, the board instantly rolls back when the handle is released, moving the food out of reach again, which guarantees that only the other members of the group present are able to get at the snack. In this way, the researchers ensure that the animal operating the handle acts purely altruistically. For the comparative behavior study with children, an analogous test apparatus was constructed, which was enclosed in a Plexiglas box and could be operated from outside by the children.
Dr. Judith Burkart
Anthropological Institute & Museum. University of Zurich
Phone +41 44 635 54 02 / +41 76 341 95 67
本文於 修改第 2 次
人類站起來的環境因素 -- AFP
E. Africa grasslands influenced human evolution: study
Grasslands dominated the cradle of humanity in east Africa longer and more broadly than thought, says a study published Thursday, bolstering the idea that the rise of such landscapes shaped human evolution.
According to the so-called "savannah hypothesis", the gradual transition from dense forests into grasslands helped drive the shift toward bipedalism, increased brain size and other distinctively human traits.
First outlined in the 1920s, the theory suggests that our most ancient upright ancestors learned to walk on two feet, in part, to peer over tall grass in search of prey and predators.
Rather than simply plucking fruit from trees, they had to become shrewd hunters and move longer distances in order to survive.
The notion has been debated for more than a century, however, with some scientists saying other forces were more important in driving humans to assume their signature posture.
They also point to studies showing that the landscapes of the two regions of east Africa richest in hominid fossils -- the Awash Valley and The Omo-Turkana Basin, both in Ethiopia -- were in fact quite diverse in terms of tree cover.
One of the most complete early hominin species yet discovered, Ardipithecus ramidus, for example, may have lived primarily inside woodlands and patches of forest, they argue.
Hominins include early humans and pre-humans, along with the early ancestors of chimpanzees and gorillas.
The new study, published in Nature, will not settle the debate, but it offers evidence that savannahs -- with their limited tree cover -- stretched back even beyond the five million-year boundary widely assumed up to now, especially in areas populated by our distant forebear.
"There have been open habitats for all of the last six million years in the environments in eastern Africa where some of the most significant early human fossils were found," said Thure Cerling, a professor at the University of Utah and lead researcher of the study.
"Wherever we find human ancestors, we find evidence for open habitats similar to savannahs -- much more open and savannah-like than forested," he said in a statement.
Combining an analysis of soil samples and satellite photos of tropical regions around the world, the researchers created vegetation chronologies for the regions home to many hominin fossils, including Ardipithecus, Australopithecus, Paranthropus and our own genus, Homo.
During the past 7.4 million years woody cover has ranged from 75 to five percent, they found.
But significant areas of savannah -- below 40 percent wood cover -- were consistently present "all the time for which we have hominin fossils in the environments where the fossils were found during the past 4.3 millions years," thus including the oldest human ancestors, Cerling said.
Up to now, many scientists believe that East Africa was forested up until two million years ago, he added.
"This study shows that during the development of bipedalism -- about four million years ago -- open conditions were present and even predominant," Cerling said.
人類行為沒有現代/原始之分 -- ScienceDaily
Earliest Humans Not So Different from Us, Research Suggests
ScienceDaily (Feb. 15, 2011) — That human evolution follows a progressive trajectory is one of the most deeply-entrenched assumptions about our species. This assumption is often expressed in popular media by showing cavemen speaking in grunts and monosyllables (the Geico Cavemen being a notable exception). But is this assumption correct? Were the earliest humans significantly different from us?
In a paper published in the latest issue of Current Anthropology, archaeologist John Shea (Stony Brook University) shows they were not.
The problem, Shea argues, is that archaeologists have been focusing on the wrong measurement of early human behavior. Archaeologists have been searching for evidence of "behavioral modernity," a quality supposedly unique to Homo sapiens, when they ought to have been investigating "behavioral variability," a quantitative dimension to the behavior of all living things.
Human origins research began in Europe, and the European Upper Paleolithic archaeological record has long been the standard against which the behavior of earlier and non-European humans is compared. During the Upper Paleolithic (45,000-12,000 years ago), Homo sapiens fossils first appear in Europe together with complex stone tool technology, carved bone tools, complex projectile weapons, advanced techniques for using fire, cave art, beads and other personal adornments. Similar behaviors are either universal or very nearly so among recent humans, and thus, archaeologists cite evidence for these behaviors as proof of human behavioral modernity.
Yet, the oldest Homo sapiens fossils occur between 100,000-200,000 years ago in Africa and southern Asia and in contexts lacking clear and consistent evidence for such behavioral modernity. For decades anthropologists contrasted these earlier "archaic" African and Asian humans with their "behaviorally-modern" Upper Paleolithic counterparts, explaining the differences between them in terms of a single "Human Revolution" that fundamentally changed human biology and behavior. Archaeologists disagree about the causes, timing, pace, and characteristics of this revolution, but there is a consensus that the behavior of the earliest Homo sapiens was significantly different than that of more-recent "modern" humans.
Shea tested the hypothesis that there were differences in behavioral variability between earlier and later Homo sapiens using stone tool evidence dating to between 250,000- 6000 years ago in eastern Africa. This region features the longest continuous archaeological record of Homo sapiens behavior. A systematic comparison of variability in stone tool making strategies over the last quarter-million years shows no single behavioral revolution in our species' evolutionary history. Instead, the evidence shows wide variability in Homo sapiens tool-making strategies from the earliest times onwards. Particular changes in stone tool technology can be explained in terms of the varying costs and benefits of different tool-making strategies, such as greater needs for cutting edge or more efficiently-transportable and functionally-versatile tools. One does not need to invoke a "human revolution" to account for these changes, they are explicable in terms of well-understood principles of behavioral ecology.
This study has important implications for archaeological research on human origins. Shea argues that comparing the behavior of our most ancient ancestors to Upper Paleolithic Europeans holistically and ranking them in terms of their "behavioral modernity" is a waste of time. There are no such things as modern humans, Shea argues, just Homo sapiens populations with a wide range of behavioral variability. Whether this range is significantly different from that of earlier and other hominin species remains to be discovered. However, the best way to advance our understanding of human behavior is by researching the sources of behavioral variability in particular adaptive strategies.
本文於 修改第 1 次