December 17, 2012
As 2012 nears its end, one thing stands out as the major theme in human evolution research this year: Our hominid ancestors were more diverse than scientists had ever imagined. Over the past 12 months, researchers have found clues indicating that throughout most of hominids’ seven-million-year history, numerous species with a range of adaptations lived at any given time. Here are my top picks for the most important discoveries this year.
1. Fossil foot reveals Lucy wasn’t alone: Lucy’s species, Australopithecus afarensis, lived roughly 3.0 million to 3.9 million years ago. So when researchers unearthed eight 3.4-million-year-old hominid foot bones in Ethiopia, they expected the fossils to belong to Lucy’s kind. The bones do indicate the creature walked upright on two legs, but the foot had an opposable big toe useful for grasping and climbing. That’s not something you see in A. afarensis feet. The researchers who analyzed the foot say it does resemble that of the 4.4-million-year-old Ardipithecus ramidus, suggesting that some type of Ardipithecus species may have been Lucy’s neighbor. But based on such few bones, it’s too soon to know what to call this species.
2. Multiple species of early Homo lived in Africa: Since the 1970s, anthropologists have debated how many species of Homo lived about two million years ago after the genus appeared in Africa. Some researchers think there were two species: Homo habilis and Homo rudolfensis; others say there was just H. habilis, a species with a lot of physical variation. It’s been a hard question to address because there’s only one well-preserved fossil, a partial skull, of the proposed species H. rudolfensis. In August, researchers working in Kenya announced they had found a lower jaw that fits with the previously found partial skull of H. rudolfensis. The new jaw doesn’t match the jaws of H. habilis, so the team concluded there must have been at least two species of Homo present.
3. New 11,500-year-old species of Homo from China: In March, researchers reported they had found a collection of hominid bones, dating to 11,500 to 14,300 years ago, in a cave in southern China. Based on the age, you’d expect the fossils to belong to Homo sapiens, but the bones have a mix of traits not seen in modern humans or populations of H. sapiens living at that time, such as a broad face and protruding jaw. That means the fossils may represent a newly discovered species of Homo that lived side by side with humans. Another possibility is that the remains came from Denisovans, a mysterious species known only from DNA extracted from the tip of a finger and a tooth. Alternatively, the collection may just reveal that H. sapiens in Asia near the end of the Pleistocene were more varied than scientists had realized.
4. Shoulder indicates A. afarensis climbed trees: Another heavily debated question in human evolution is whether early hominids still climbed trees even though they were built for upright walking on the ground. Fossilized shoulder blades of a 3.3-million-year-old A. afarensis child suggest the answer is yes. Scientists compared the shoulders to those of adult A. afarensis specimens, as well as those of modern humans and apes. The team determined that the A. afarensis shoulder underwent developmental changes during childhood that resemble those of chimps, whose shoulder growth is affected by the act of climbing. The similar growth patterns hint that A. afarensis, at least the youngsters, spent part of their time in trees.
5. Earliest projectile weapons unearthed: Archaeologists made two big discoveries this year related to projectile technology. At the Kathu Pan 1 site in South Africa, archaeologists recovered 500,000-year-old stone points that hominids used to make the earliest known spears. Some 300,000 years later, humans had started making spear-throwers and maybe even bow and arrows. At the South African site called Pinnacle Point, another group of researchers uncovered tiny stone tips dated to 71,000 years ago that were likely used to make such projectile weapons. The geological record indicates early humans made these small tips over thousands of years, suggesting people at this point had the cognitive and linguistic abilities to pass on instructions to make complex tools over hundreds of generations.
6. Oldest evidence of modern culture: The timing and pattern of the emergence of modern human culture is yet another hotly contested area of paleoanthropology. Some researchers think the development of modern behavior was a long, gradual buildup while others see it as progressing in fits and starts. In August, archaeologists contributed new evidence to the debate. At South Africa’s Border Cave, a team unearthed a collection of 44,000-year-old artifacts, including bone awls, beads, digging sticks and hafting resin, that resemble tools used by modern San culture today. The archaeologists say this is the oldest instance of modern culture, that is, the oldest set of tools that match those used by living people.
7. Earliest example of hominid fire: Studying the origins of fire is difficult because it’s often hard to differentiate a natural fire that hominids might have taken advantage of versus a fire that our ancestors actually ignited. Claims for early controlled fires go back almost two million years. In April, researchers announced they had established the most “secure” evidence of hominids starting blazes: one-million-year-old charred bones and plant remains from a cave in South Africa. Because the fire occurred in a cave, hominids are the most likely cause of the inferno, the researchers say.
8. Human-Neanderthal matings dated: It’s not news that Neanderthals and H. sapiens mated with each other, as Neanderthal DNA makes up a small portion of the human genome. But this year scientists estimated when these trysts took place: 47,000 to 65,000 years ago. The timing makes sense; it coincides with the period when humans were thought to have left Africa and spread into Asia and Europe.
9. Australopithecus sediba dined on wood: Food particles stuck on the teeth of a fossil of A. sediba revealed the nearly two-million-year-old hominid ate wood—something not yet found in any other hominid species. A. sediba was found in South Africa in 2010 and is a candidate for ancestor of the genus Homo.
10. Earliest H. sapiens fossils from Southeast Asia: Scientists working in a cave in Laos dug up fossils dating to between 46,000 and 63,000 years ago. Several aspects of the bones, including a widening of the skull behind the eyes, indicate the bones were of H. sapiens. Although other potential modern human fossils in Southeast Asia are older than this find, the researchers claim the remains from Laos are the most conclusive evidence of early humans in the region.
September 26, 2012
By 200,000 years ago, Homo sapiens had emerged somewhere in Africa. By 14,000 years ago, our species had spread to every continent except Antarctica. What happened in between—the pattern of where humans went and when—is still being worked out. To reconstruct the peopling of the world, anthropologists rely on several types of clues.
Fossils: The most obvious way to track our ancestors’ movements is to look for their physical remains. Researchers sketch out travel routes by mapping where the oldest human fossils are found. The earliest Homo sapiens bones outside of Africa come from a cave site in Israel called Qafzeh. Here the skeletons of both adults and children date to as far as 125,000 years ago. This first foray out of Africa didn’t last long. Humans disappeared from the fossil record outside of Africa for many tens of thousands of years, perhaps because the climate became too harsh. Fossils tell us humans made a successful, sustained exodus by at least 50,000 years ago. Human fossils found at Australia’s Lake Mungo site, for example, have been dated to between 46,000 and 50,000 years ago (PDF).
The problem with relying on skeletal remains to map early migrations is that the timing of our ancestors’ travels is only as good as the methods used to date the fossils. Sometimes scientists find bones in places that are not easily dated by geological techniques. And in some areas, fossils aren’t prone to preservation, so there are probably huge gaps in our knowledge of the paths early humans took as they spread around the world.
Artifacts: Archaeologists also look for the items people made and left behind. For example, stone tool discoveries suggest an alternative route out of Africa. For decades, scientists assumed humans left Africa via the Sinai Peninsula, but in the last several years some researchers have favored a “southern” route: leaving from the Horn of Africa, crossing the narrowest part of the Red Sea and entering into southern Arabia. Last year, archaeologists reported finding stone tools in Oman dating to roughly 106,000 years ago. At that time, the Arabian Peninsula was a much more hospitable place than it is today, home to numerous freshwater lakes. As the region became drier, people might have moved east into Asia or returned to Africa.
Of course, when the only remains at an archaeological site are tools, it’s hard to say with absolute certainty who made them. The researchers working in Oman noted that the tools they found in Arabia match the technology of modern humans found in eastern Africa about 128,000 years ago. The team made the case that the tool makers on either side of the Red Sea belonged to the same cultural group—and therefore the same species. But as anthropologists discover more species, such as the Hobbit or the Denisovans, that lived alongside modern humans outside of Africa up until a few tens of thousands of years ago, it becomes harder to say stone tools alone indicate the presence of Homo sapiens.
DNA: Genetic data can help fill in the holes in the human migration story that fossils and artifacts can’t address. Anthropologists collect DNA samples from different ethnic groups around the world. Next, they count up the genetic differences caused by mutations in certain sections of the genome. Groups that are more closely related will have fewer genetic differences, which implies they split off more recently form each other than they did with more distantly related groups. Scientists calculate when in the past different groups diverged from each other by adding up all of the genetic differences between two groups and then estimating how often genetic mutations occurred. Such analyses not only give a sense of when different parts of the world were first inhabited, but they can also reveal more intricate patterns of movement. For example, genetic data suggest North America was colonized by three separate waves of people leaving Siberia across the Bering Strait.
Genetic data are not foolproof, however. The estimated divergence times are only as accurate as the estimated mutation rate, which scientists still debate. In the early days of DNA studies, scientists used either mitochondrial DNA, passed down only by the mother, or the Y chromosome, inherited only from father to son. Neither of these types of DNA presented the full picture of what people were doing in the past, as mitochondrial DNA only tracks maternal lineages while the Y chromosome only follows paternal lines. Today, whole genome sequencing is beginning to allow researchers to trace entire populations.
Languages: Anthropologists use languages in methods analogous to studying DNA; they look for patterns of similarities, or differences, in vocabularies or other aspects of language. Earlier this year, researchers compared different languages within the Indo-European language family to determine where these languages arose. After assessing the relationship between the languages, the researchers considered the geographic ranges where those languages are currently spoken. They concluded that the Indo-European language family originated in what is today Turkey and then spread west into Europe and east into southern Asia as people moved into these areas. But such linguistic analyses may only track relatively recent migration patterns. For example, H. Craig Melchert, a linguist at the University of California, Los Angeles, told Science News that the Indo-European languages can only be traced back about 7,000 years.
August 15, 2012
Two years ago the analysis of the Neanderthal genome revealed modern humans carry Neanderthal DNA, implying our ancestors mated with Neanderthals at some point in the past. Scientists only found genetic traces of Neanderthals in non-African people, leading to the conclusion that Neanderthal-human matings must have occurred as modern humans left Africa and populated the rest of the world. A new paper (PDF) posted on arXiv.org puts a date on those matings: 47,000 to 65,000 years ago—a time that does indeed correspond with human migrations out of Africa.
Sriram Sankararaman of Harvard Medical School and colleagues—including Svante Pääbo of Germany’s Max Planck Institute for Evolutionary Anthropology and Harvard’s David Reich—investigated the timing of the matings in part to verify that the trysts even happened at all. That’s because there’s an alternative explanation for why up to 4 percent of non-African human DNA looks like Neanderthal DNA. It’s possible, the researchers explain, that the ancestral species that gave rise to both humans and Neanderthals had a genetically subdivided population—in other words, genetic variation wasn’t evenly distributed across the species. Under that scenario, Neanderthals and the modern humans that left Africa might have independently inherited similar DNA from a part of the divided ancestral population that didn’t contribute genetic material to modern African populations. (Another paper published this week, in Proceedings of the National Academy of Sciences, considers this scenario.)
To determine what really happened, Sankararaman’s team looked at rates of genetic change to estimate when Neanderthals and humans last exchanged genes. If the shared DNA was due to interbreeding, the team expected to find a date less than 100,000 years ago—some time after humans left Africa. But if it was the result of sharing a common ancestor, they expected a date older than 230,000 years ago, approximately when Neanderthals and modern humans split from each other. The team’s findings support the interbreeding scenario: 47,000 to 65,000 years ago.
Neanderthals aren’t the only archaic species that may have contributed to the modern human gene pool. Denisovans, known from only a tooth and a finger bone, left a genetic mark in people living in Melanesia and Southeast Asia. And recent genetic evidence suggests that some ancient African populations mated with an unidentified, now-extinct hominid species that lived in Africa.
So far, our knowledge of Neanderthal and Denisovan genetics comes from only a few individuals, so our understanding of interspecies mating is likely to change as more Neanderthal and Denisovan DNA is analyzed.
(H/T John Hawks)
July 17, 2012
A hundred years ago, archaeologists thought Native Americans came to North America only 5,000 years ago. That belief changed in the 1920s and 1930s as researchers started finding stone projectile points associated with the fossils of mammoths and giant bisons—animals that went extinct more than 10,000 years ago. For decades, the oldest known points dated to 13,000 years ago. Called Clovis points, they contained characteristic “flutes,” or long, concave grooves, where a spear locked into place.
More recent evidence reveals humans reached the New World, via the Bering Strait, by at least 15,000 years ago. These early Americans weren’t making Clovis points. Last week, archaeologists announced in Science another example of pre-Clovis technology.
The tools come from Oregon’s Paisley Caves. Dennis Jenkins of the University of Oregon and colleagues determined people were living in the area by at least 14,000 years ago based on the radiocarbon dates of human coprolites (fossilized dung) found in the cave. They also found projectile points of the same age or slightly older than Clovis points. Known as the Western Stemmed Tradition, these points are narrower, lack flutes and require a different chipping method to make than Clovis points.
The team suggests the Clovis and Western Stemmed points probably developed independently from an even earlier tool technology, with the Clovis originating in the Plains and Southeast and the Western Stemmed arising in the West. This fits with a discovery reported last year in Science. At the Debra L. Friedkin site in central Texas, archaeologists recovered more than 16,000 artifacts dating to 13,200 to 15,500 years ago. Among the artifacts were blades and two-sided flakes that Clovis tools could have evolved from, the researchers suggested. (A study published online in the Journal of Archaeological Science, however, challenges those dates and even argues that the artifacts may actually be Clovis tools.)
In other early American news, a team led by David Reich of Harvard Medical School reconstructed the ancestry of Native Americans living in North and South America. They reported their findings last week in Nature. Based on a genetic analysis of 52 modern Native American groups and 17 Siberian groups, the researchers concluded the majority of Native Americans descend from a single Siberian population. Arctic people who speak Eskimo-Aleut languages also inherited about half of their genetic material from a second wave of Siberian immigrants. Members of a third migration contributed to the gene pool of Na-Dene-speaking Chipewyans of Canada. Finding multiple migrations complements previous genetic, archaeological and linguistic studies.
Of course, that doesn’t mean there were only three migrations to the New World. The researchers only looked at the ancestry of living Native Americans. There could be early migrating groups that didn’t leave behind living descendants. That’s something we may never know.
July 9, 2012
The 2003 discovery of the diminutive Homo floresiensis, better known as the Hobbit, on the Indonesian island of Flores was a shock. Anthropologists never expected to find a 3-foot, 6-inch-tall hominid living in Southeast Asia at the same time as modern humans, as recently as 17,000 years ago. Aside from the controversy over the hominid’s true identity—a diseased Homo sapiens or a member of its own species—another intriguing question was how the ancestors of the Hobbits got to Flores.
One possibility is that the Hobbits’ forefathers sailed over on a raft. Or their arrival might have been an act of nature: A powerful storm or tsunami could have washed a small group of hominids out to sea, and then floating vegetation carried them to Flores. That idea sounds implausible, but it’s also an explanation for how monkeys reached South America.
Scientists will probably never know for certain what the Hobbit’s ancestors went through to get to Flores. Such ancient wooden boats are unlikely to be preserved and there’s no way to prove it was a freak accident.
But recently a pair of researchers offered a novel way of assessing the issue. Ecologist Graeme Ruxton of the University of St. Andrews in Scotland and biologist David Wilkinson of Liverpool John Moores University in England simulated population growth over time of planned colonizations versus accidental castaways to see which scenario could lead to successful inhabitations of an island. They reported their results in the Journal of Human Evolution.
The premise of the model is that a group of hominids reach an island. The hominids mate monogamously and each year there is a set probability that a female of a certain age will give birth. There’s also a given probability that individuals in the population will die, based on age and sex.
For the scenario of a planned trip at sea aboard a raft, Ruxton and Wilkinson assumed colonists were sailing as groups of families. So the founding populations in this model had an equal number of adult males and adult females. Ruxton and Wilkinson ran their simulations using different group sizes for a founding population. After running each scenario a thousand different times, they concluded such populations could be successful—defined as lasting 500 years or reaching 500 individuals. The likelihood of success increased with founding population group size, reaching a success plateau at groups of just 20.
To simulate an accidental island arrival due to a storm or tsunami, the pair changed one of their starting assumptions. Instead having an equal number of adult males and adult females at the onset, they assumed the sex ratio was random. No one plans to be washed out to sea, after all. Under this scenario, colonizations were 50 percent less likely to succeed compared to the planned trips aboard a boat. But with slight modifications, that number went up. By adding a 2 percent chance that one to four additional castaways might reach the island each year for the first 400 years, Ruxton and Wilkinson found that unintentional colonizations were as likely to succeed as planned ones. These newcomers increased a stranded population’s chance of long-term viability by introducing new genes to the island and/or balancing out skewed sex or age ratios.
Although the chance of different storms washing different groups of hominids to the same island sounds as likely as lightning striking twice, it may not be that far-fetched. Ruxton and Wilkinson point out ocean currents and wind patterns can lead floating objects to the same place over and over again.
So what does all of this computation really tell us? On the one hand, the models are only as useful as the assumptions Ruxton and Wilkinson used to build them. If the hominids didn’t mate monogamously, for example, then the pair’s conclusions may not be valid. But putting such concerns aside, the results indicate that both rafting and accidental ocean dispersals are possible explanations for the Hobbits’ inhabitation of Flores. Therefore, the researchers warn, a hominid’s presence on an island isn’t necessarily evidence of some kind of sailing technology.
Today, humans live on tens of thousands of islands—even if they didn’t necessarily mean to.