Anatomically Modern H. sapiens
- THE MODERNS
- The first anatomically modern H. sapiens appear in the fossil record soon after 100,000 years ago. They are much more gracile than earlier groups (e.g., Neandertals and other archaics) and share a number of physical characteristics with modern human populations, including:
Smaller face, with prominent chin
Rounded skull
Increasingly gracile postcranial morphology
Interaction between Neandertals and anatomically modern humans (AMH) has been suggested on the basis of findings in the Levant (Middle East). Two of the oldest AMH sites - Skhul and Qafzeh - were discovered in close spatial (and overlapping temporal) proximity to Neandertal sites (Tabun, Amud, and Kebara).
- Why Are Our Brains So Big?
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Our brains are not just big—they're grotesquely huge. A typical mammal our size would have a brain one-seventh as large as ours. And big brains are relatively new for hominids. From 7 million to 2 million years ago, our ancestors had brains about the size of a modern chimpanzee's.
Hominid brains only began to increase 2 million years ago, and they continued to balloon, in fits and starts, until they neared their present size at least 160,000 years ago.
When it comes to explaining this explosion in brain size, scientists agree on one thing: It must have offered a powerful evolutionary advantage.
"It costs you an awful lot in terms of energy," says Aiello. "You don't evolve large and expensive organs unless there's a reason."
But paleoanthropologists are divided about that reason. One possibility is that bigger brains gave hominids extra information-processing power they could use to make better tools. After all, stone tools unlocked new supplies of food, and so better tool users could support more offspring. Another possibility is that the driving force was hominid social life. Primates living in big groups tend to have bigger brains, possibly because there's an evolutionary advantage to keeping track of other members of your group. And certainly the human brain has evolved into an awesome social computer, able to draw subtle clues about other people's thoughts from their faces in a fraction of a second.
On the other hand, big brains may have prompted humans to become more social. For one thing, big brains made children helpless. Hominid kids, then as now, needed years to develop large brains, during which time they depended on adults for high-energy foods. It's possible that the basic shape of the human family as a group of parents, siblings, and grandparents formed to feed the brains of their children.
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- How Did We Get Modern Minds?
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Walking upright, growing a big brain, and even making tools are not enough to make an ape truly human. Consider Homo ergaster, a species that lived in Africa between 1.7 million and 600,000 years ago and probably gave rise to our own species. H. ergaster stood up to six feet tall, had a medium-size brain, and could survive even in arid grasslands thanks to an impressive kit of stone axes and other tools.
Despite all that, this species' brain didn't work like ours. For hundreds of thousands of years, H. ergaster was content to use the same set of tools, with few modifications. Putting a stone axe on the end of a stick to make a spear would have allowed these hominids to become much better hunters, and yet this simple idea apparently never occurred to them. Such an idea seems simple only to our modern minds, which can see new possibilities in the world, discover hidden connections, and think and communicate with symbols.
Scientists don't yet know how that modern mind came into existence. The question is particularly hard to answer because they can't get into the brain of H. ergaster or any of our other ancestors. Instead, they have to infer what those ancient minds were like by looking at the things they made. The people who painted pictures of mammoths and woolly rhinos in French caves almost 32,000 years ago must have already had minds much like our own. Archaeologists have documented an explosion of expressions of the modern mind after roughly 50,000 years ago, in the form of jewelry, elaborate graves, bone-tipped spears, and other new kinds of tools.
The bones of the people who made these things look like our own. They were members of Homo sapiens, complete with long, slender arms and legs, a flat face, a jutting chin, and a high forehead that fronted a big brain. But they were hardly the first people with our anatomy. H. sapiens fossils have been found in Africa from at least 160,000 years ago, and some experts argue that the earliest members of our species may have existed over 200,000 years ago.
Richard Klein, a paleoanthropologist at Stanford University, has offered a controversial theory: The modern mind is the result of a rapid genetic change. He puts the date of the change at around 50,000 years ago, pointing out that the rise of cultural artifacts comes after that date, as does the spread of modern humans from Africa. The evolution of the modern mind allowed humans to thrive as never before, Klein argues, and soon even a continent as huge as Africa could not contain their expanding population.
Many other paleoanthropologists beg to differ. Sally McBrearty, an archaeologist at the University of Connecticut, believes the evidence shows that the technology and artistic expression of modern humans emerged slowly over hundreds of thousands of years, as humans gradually moved into new habitats and increased their population. She points to a long list of tantalizing clues in Africa that predate Klein's 50,000-year milestone.
Humans may have been grinding pigments 250,000 years ago, for example, and researchers have found barbed bone fishing hooks in Central Africa that they estimate are 90,000 years old. Last year (in 2002) scientists in South Africa discovered stones covered with geometrical cross-hatching dating back 77,000 years.
Klein dismisses the evidence for such slow-fuse change as paltry and misleading. "It's a little bit here, it's a little bit there. Most sites don't have anything like this at all, but when you get to 50,000 years ago, they all do. Then you get real art—not stuff you can argue about whether it shows some form of symbolism—and elaborate graves and houses and the rest of it."
A resolution to this debate may be waiting in Africa, at archaeological sites scattered across the continent. "We know what we'd like to find and where we ought to look for it," says McBrearty. "But are we going to have the money and the perseverance to mount the assault and come up with the goods?"
- Upper Paleolithic
- Upper Paleolithic technology, culture, and behavior begins to appear in the archaeological record sometime after 45 kyr. Some of these differences include:
larger, more elaborate (specialized) tool kit
use of bone, antler, and other materials, in addition to stone
production of ornamental artifacts
construction of elaborate shelters
increased use of exotic (non-local) raw material in toolmaking
The first anatomically modern humans found in Europe were called the Cro Magnons (general cranial traits pictured above). While the individuals found at Cro Magnon in particular were not the earliest inhabitants of Europe, it appears as though people very similar to the Cro Magnons precipitated the sudden disappearance of Europe's previous inhabitants, the Neandertals; though, once again, the actual nature of these events remains a mystery.
Upper Paleolithic technologies spread to many regions, including Australia, by 45 kyr. Upper Paleolithic technology is not found in southern Asia until many years later (at about 12 kyr). It remains a possibility that undiscovered sites exist in the region, since Upper Paleolithic industries inevitably passed through the region. However, crude stone tools remain common in southern Asia, and recent redating of H. erectus fossils on Java suggest that they may have existed isolated on the island until as recent as 27 kyr.
- Modern Behavior
- Modern humans utilizing Upper Paleolithic technology continued to live in a manner not unlike that of Neandertals; however, they were able to exploit a wider range of resources and different environments. This technological advantage is expected to have manifested itself in the demographics of Upper Paleolithic cultures:
Upper Paleolithic modern humans lived at much higher population densities than Neandertals
they also lived longer than Neandertals
they incurred fewer instances of injury and disease
What then, is the basis of this sudden modernization which became such a successful strategy for modern humans? One argument suggests that the change began at the genetic level, creating a biological template that facilitated modern behavior. These changes would be indubitably cognitive in nature, and are not likely to materialize in the fossil record. However, another argument suggests that the human revolution came as a result of cultural evolution, and that the hiatus between the emergence of anatomically modern humans and the "appropriate" behavior was merely a period of cultural development and accumulation.
- The Lagar Velho 1 Skeleton
- In April 1999, the discovery of a human skeleton from Lagar Velho in Portugal was announced in the media, followed by a scientific paper a couple of months later (Duarte et al. 1999). The skeleton is of a young boy, about 4 years in age, who was deliberately buried about 24,500 years ago.
According to the paper's authors, which included Neandertal expert Erik Trinkaus, the skeleton contains a mixture of features from both modern humans and Neandertals, and is best explained as being a hybrid. And because it is dated to be at least 4,000 years more recent than the last known Neandertals, they consider it to be not the result of a direct interbreeding, but the descendant of a hybrid population which persisted for thousands of years. If true, this would strongly support the claim that Neandertals should be considered a subspecies of modern humans (Homo sapiens neanderthalensis), rather than a separate species, Homo neanderthalensis.
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- The Nariokotome Boy
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The Nariokotome boy is a remarkably complete skeleton of Homo erectus (also sometimes called Homo ergaster) and illustrates many of the evolutionary developments that distinguish the early humans from the australopiths that preceded them. These include a skeleton more specialized for bipedalism and a larger brain.
About 1.8 million years ago, a boy died. An abcessed tooth suggests an infection may have killed him, although the cause is not certain. His bones were fossilized, and they lay undetected until 1985, when they were discovered by paleoanthropologist Richard Leakey.
The Nariokotome boy's skeleton has been called the fossil find of the century. Because it was so complete, it revealed a great deal not only about the anatomy of his species, Homo erectus (sometimes called Homo ergaster), but also clues about their life history and even social structure.
He was only about 10 years old when he died, revealed by the fact that his molar teeth were still emerging, yet he was already about five feet tall and would have been over six feet at maturity. His legs were relatively long in proportion to his body compared to earlier hominids like Lucy.
With his tall, slender build, he was well adapted to staying cool in hot, dry climates. His ribcage and shoulder girdle indicate that he could swing his arms when walking or running, as we do, something that earlier hominids were incapable of. Combined with his relatively slender waist, which gave him more flexibility, this suggests an adaptation to greater speed or endurance, or both.
His face, molar teeth, and chewing muscles are smaller than those of earlier hominids, suggesting a softer, high-quality diet, which he would have needed to nourish his relatively large brain. (With a volume of about 880 cubic centimeters, his cranium was larger than an australopithecine's, but smaller than a modern human's.) The brain is a very costly organ metabolically, and needs a reliable source of both calories and protein to sustain it.
The proportions of skull to pelvis indicates that his sisters would have had to give birth to relatively immature infants, to allow the baby's skull to fit through the pelvic opening. These immature infants, much more helpless than, say, a newborn chimp, would have needed an extended period of care, more like a modern human baby. This in turn suggests that his species must have been able to provide the necessary support for a mother and her child, whether in the form of monogamous pair bonding or some other social system.
With the appearance Homo erectus/ergaster in the fossil record we also see the first use of fire, the first appearance of more systematic toolmaking, and the first migration of hominids outside Africa. The Nariokotome boy and his kind represent the beginning of a new phase of human prehistory.
- Humans and Chimps
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A lot more genes may separate humans from their chimp relatives than earlier studies let on. Researchers studying changes in the number of copies of genes in the two species found that their mix of genes is only 94 percent identical. The 6 percent difference is considerably larger than the commonly cited figure of 1.5 percent.
The new finding supports the idea that evolution may have given humans new genes with new functions that don't exist in chimps, something researchers had not recognized until recently. The older value of 1.5 percent is a measure of the difference between equivalent genes in humans and chimps, like a difference in the spelling of the same word in two similar languages. Based on that figure, experts proposed that humans and chimps have essentially the same genes, but differed in when and where the genes turn on and off.
The new research takes into account the possibility for multiple copies of genes and that the number of copies can differ between species, even though the gene itself is the same or nearly so. "You have to pay attention to more than just the genes that are shared," says geneticist Matthew Hahn of Indiana University, Bloomington, lead author of the new report. Researchers believe that additional copies of the same gene allow evolution to experiment, so to speak, finding new functions for old genes.
Hahn and his colleagues set out to study these gains and losses in gene number over the millennia by examining the genomes of humans, chimps, mice, rats and dogs. They looked at 110,000 genes that fall into 9,990 different families of similar genes.
The size of a gene family differed between species in 5,622 cases, or 56 percent of all the families. These size changes are so frequent in the evolutionary history of mammals that genes might as well be going through a revolving door.
In humans and chimps, which have about 22,000 genes each, the group found 1,418 duplicates that one or the other does not possess. For example, humans have 15 members of a family of brain genes linked to autism, called the centaurin-gamma family, whereas chimps have six, for a difference of nine gene copies.
The group estimated that humans have acquired 689 new gene duplicates and lost 86 since diverging from our common ancestor with chimps six million years ago. Similarly, they reckoned that chimps have lost 729 gene copies that humans still have.
"The paper supports the emerging view that change in gene copy number, via gene duplication or loss, is one of the key mechanisms driving mammalian evolution," says genomics researcher James Sikela of the University of Colorado Health Sciences Center.
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