August 22, 2012
Hominid hunting requires a lot of hard work and determination. Paleoanthropologists can spend months surveying a landscape, studying the fine details of a geologic formation and sifting through mounds of sediments. But sometimes all it takes is dumb luck. Here’s a look at five hominid fossil discoveries that were complete accidents.
Neanderthal 1 (1856): While quarrying limestone, workers unearthed some bones in Feldhofer Cave in Germany’s Neander Valley. The men thought they had found the remains of an old bear and tossed the fossils aside. The quarry’s owner gave one of the bones, a skullcap, to schoolteacher Johann Fuhlrott. Although the skull had thick browridges and a sloping forehead, Fuhlrott recognized the fossil was more human than bear and turned it over to Hermann Schaffhausen, an anatomist at the University of Bonn who concluded the skull belonged to an ancient human race. In 1864, Irish geologist William King pointed out that the cave sediments in which the fossil was found dated to more than 30,000 years ago. Due to the great antiquity, he suggested the skullcap belonged to an extinct species of human, one that he named Homo neanderthalensis. This was the first time anyone had recognized a fossil as being a part of an extinct hominid species. But Neanderthal 1, as the skullcap is now called, wasn’t the first Neanderthal ever found. A skull discovered in Belgium in 1829 and another one found in Gibraltar in 1848 were later classified as Neanderthals.
Cro-Magnon (1868): Clearing a path for a road in southern France, construction workers exposed the entrance to a limestone rock shelter. The cave was named Cro-Magnon and inside workers found the skeletons of four adult Homo sapiens and one infant, in addition to stone tools and perforated shell beads. Researchers realized these humans were quite old because their bones were found in association with the remains of mammoths and lions. (Radiocarbon dating in the 1950s confirmed that these people lived roughly 30,000 years ago.) The name Cro-Magnon eventually became synonymous with early Europeans from this time period.
Kabwe 1 (1921): At the Broken Hill (now Kabwe) iron and zinc mine in Zambia, Swiss miner Tom Zwiglaar came across several fossils, including a skull, jaw and leg bones. The specimens looked human, but the skull also had features that didn’t resemble any modern people, such as heart-shaped browridges and a sloping forehead. The bones were sent to British paleontologist Arthur Smith Woodward. He decided the fossils represented an extinct hominid species he called Homo rhodesiensis (Zambia was once part of the British colony Northern Rhodesia). Today, the Kabwe 1 skull, dating to 300,000 to 125,000 years ago, is classified in the species Homo heidelbergensis, which some paleoanthropologists think was the common ancestor of Neanderthals and modern humans.
Taung Child (1924): Clearly, mines are a great place to stumble across hominid fossils. The discovery of the Taung Child is no exception. In 1924, a mining official noticed a monkey skull lodged in a chunk of limestone that had been blasted from a quarry near Taung, South Africa. The official brought the skull home, and his son later showed to it Raymond Dart, an anatomy professor at the University of the Witwatersrand. Intrigued by the specimen, Dart had the quarry send over some more rubble that might contain fossils. Inside was a promising rock that looked like the surface of a brain. Careful scraping with a pair of knitting needles allowed Dart to liberate the brain’s corresponding face from another piece of rock. The face looked like an ape, but Dart recognized that aspects of its brain looked like a human’s. He believed the fossil represented an intermediate species between apes and humans, and named it Australopithecus africanus. It was the first discovery of an Australopithecus, and it spurred other hominid hunters to start looking for our ancestors in Africa.
Australopithecus sediba (2008): This discovery wasn’t completely unexpected, but the finder of the fossil was. Lee Berger of the University of the Witwatersrand was surveying South Africa’s Malapa Cave with his Witwatersrand colleague Job Kibii when Berger’s 9-year-old son Matthew announced he had found something: a rock with a hominid collar bone sticking out. Additional excavation led to the recovery of two hominid skeletons dating to nearly two million years ago. The older Berger decided the skeletons represented a new species, Australopithecus sediba, which is a leading candidate for ancestor of the genus Homo.
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 25, 2012
The London Olympics are a great excuse to talk about England’s hominid history. Current evidence suggests that hominids reached Great Britain by at least 800,000 years ago, when the island was connected to mainland Europe. Since then, as many as four different hominid species have lived there. Coming and going in response to climate change, hominids probably fled England during extreme cold times when glacial ice covered the area. Sometime between 450,000 and 200,000 years ago, catastrophic flooding of a glacial lake eroded the land bridge connecting Great Britain and Europe and changed the drainage patterns of the region’s rivers. As a consequence, during warm periods when polar ice sheets melted and sea levels rose, the land bridge was transformed into a channel. This barrier probably explains why hominids are absent from the fossil record 180,000 to 60,000 years ago. It wasn’t until 12,000 years ago that the ancestors of modern Brits finally arrived on the island and stayed for good.
With that mini-review in mind, here are five of England’s most important human evolution discoveries.
Happisburgh (~780,000 years ago): This site, about a three-hour drive northeast of London, contains England’s earliest evidence of hominids. In 2010, archaeologists announced in the journal Nature that they had found flaked stone tools dating to between 990,000 and 780,000 years ago, when Great Britain was connected to mainland Europe. Fossils and climate data suggest the environment was much like modern southern Scandinavia, home to coniferous forests. No hominid fossils have been found there yet. But back in 2010, paleoanthropologist Chris Stringer of the Natural History Museum in London told Nature News that these hominids might have been members of the lesser-known species Homo antecessor.
Pakefield (700,000 years ago): Before the discoveries at Happisburgh, this was the oldest archaeological site in England. About an hour south of Happisburgh, the younger Pakefield find consists of more than 30 stone tools, and the environmental data suggests the hominids here experienced a warm, seasonally dry Mediterranean climate, researchers reported in Nature in 2005.
Boxgrove (500,000 years ago): On England’s southern coast in the 1990s, anthropologists recovered what are the oldest hominid remains ever found in that country: a shin bone and two teeth dating to half a million years ago. Researchers think the bones belonged to Homo heidelbergensis, the species that many anthropologists consider to be the common ancestor of modern humans and Neanderthals. Stone tools and fossils at the site reveal the hominids butchered horses, deer and rhinos. Wolves, lions and hyenas also lived nearby (PDF).
Swanscombe (400,000 years ago): Between 1933 and 1955, amateur archaeologists discovered three separate pieces of the same female skull at a gravel quarry in Swanscombe. The skull is thought to be that of an early Neanderthal (although the skull’s age and species status have been questioned.) Less than an hour east of London, the Swanscombe site is now a historical park.
Kent’s Cavern (~41,000 years ago): In 2011, researchers reanalyzed a partial upper jaw and teeth discovered in 1927 in Kent’s Cavern in southwestern England. Originally thought to be 35,000 years old, the fossils are actually about 41,000 years old, the researchers reported in Nature. The older date makes these the oldest modern human (Homo sapiens) bones found in England and among the oldest ever found in Europe. Today, tourists can visit the cavern (and even get married there).
If this isn’t enough British hominid history for you, try reading Chris Stringer’s Homo britannicus.
July 23, 2012
Modern humans, Homo sapiens, originated in Africa sometime between 200,000 and 100,000 years ago. I’ve written that sentence many times. But what if it’s wrong? Paleoanthropologist Tim Weaver of the University of California, Davis argues there might be another way to interpret our species’ beginnings. Instead of a discrete origin event, he suggests in the Journal of Human Evolution that our ancestors’ arrival into the world might have been a lengthy process that occurred over hundreds of thousands of years.
Current thinking says the lineages leading to modern humans and Neanderthals split 400,000 years ago. And then 200,000 years later, Homo sapiens suddenly appeared in Africa. There’s a lot of evidence that seems to support the idea. The earliest fossils assigned to our species date to this time period. Mitochondrial DNA inherited through the maternal line backs up the fossil evidence. Modern people’s mitochondrial DNA can all be traced back to a common ancestor, an “Eve,” that lived 200,000 years ago.
But Weaver says these lines of evidence can also support an alternative scenario, in which the evolution of our species plays out over hundreds of thousands of years between the split from Neanderthals and the expansion of humans out of Africa 60,000 to 50,000 years ago. He uses genetics and mathematical methods to argue his case.
First, he shows how modern people’s mitochondrial DNA could all appear to converge at 200,000 years ago without being the result of a speciation event or a population bottleneck at that time. It’s possible, he says, to get the same picture of modern mitochondrial DNA if the population of breeding adults stayed constant 400,000 to 50,000 years ago—and if the size of that population equaled the average (called the harmonic mean) population size of the successive generations experiencing a theoretical bottleneck 200,000 years ago.
Next, he builds a model of physical evolution to show how a long process could lead to the arrival of modern human traits at about 200,000 years ago. The model follows several assumptions about the genetic basis of physical traits. Weaver also assumes changes over time in human physical traits were the result of mutation and genetic drift (random change) rather than natural selection. (He notes that differences between Neanderthal and modern human skulls, for example, don’t appear to be the result of natural selection.) By modeling successive generations from 400,000 years ago to the present, with each generation equaling 25 years, Weaver finds modern human traits should have appeared in the fossil record 165,000 years ago. That date becomes 198,000 years ago when the generation length is increased to 30 years or 132,000 years ago when the generation length is decreased to 20 years. What that means is both an abrupt speciation event or a long process could explain why modern humans seem to appear in the fossil record 200,000 years ago.
Weaver’s purpose with this work, however, is not necessarily to prove that modern human origins was a long, drawn out affair. He writes:
At the moment, both discrete event and lengthy process models appear to be compatible with the available evidence. My goal is simply to show that lengthy process models are consistent with current biological evidence and to heighten awareness of the implications of these models for understanding modern human origins.
One of those implications: If it turns out the arrival of humans was a lengthy process, Weaver says, it means nothing “special” happened 200,000 years ago to cause the birth of our species.
July 18, 2012
Neanderthals didn’t ride bucking broncos (as far as we know), but the Stone Age hominids did seem to have one thing in common with rodeo riders: injuries. In 1995, paleoanthropologists Thomas Berger and Erik Trinkaus, now at Washington University in St. Louis, noted that Neanderthals had a disproportionate number of injuries to their heads and necks. The same is true among modern rodeo riders. Just as these cowboys get too close for comfort to angry horses and bulls, Neanderthals’ hunting style—sneaking up on prey and jabbing them with heavy spears—brought their upper bodies within striking distance of large, hoofed animals.
Over the last 17 years, researchers have reassessed the Neanderthal-rodeo rider connection. Recently, in the Journal of Archaeological Science, Trinkaus offered alternative explanations for the trauma patterns.
In the new study, Trinkaus considered the injuries recorded in the bones of early modern humans that lived at the same time as Neanderthals. Early human trauma hadn’t been as well studied as Neanderthal trauma. Statistically speaking, Trinkaus saw no difference between the two species’ wounds; they both suffered a lot of harm to the head and neck. This means ambush hunting may not account for all of these injuries because humans often hurled projectiles at animals while standing back at a safe distance. Recent archaeological work indicates Neanderthals might have done the same thing on occasion. Instead, the source of those injuries might have been violent attacks within or between the two species.
Then again, Trinkaus suggests, Neanderthals and humans might not have had an abnormal amount of upper body trauma after all. He points out that even minor injuries to the head can leave marks on the skull because there isn’t a lot of tissue separating the skin and bone. Arms and legs, however, have fat and muscle that safeguard the bones against more minor flesh wounds. So, anthropologists may not be getting a good estimate of trauma to these parts of the body.
Another factor might also be masking lower body injuries—the mobile lifestyle of Stone Age hominids. Both humans and Neanderthals moved around a lot to find appropriate food and shelter. An individual who couldn’t keep up with the group, due to a broken leg, say, might have been left behind to die, perhaps in places where their bones didn’t readily preserve. (Trinkaus acknowledges that some fossils of old, sick Neanderthals have been found. But although their afflictions, such as arthritis, would have been painful, they wouldn’t have prevented them from walking.)
As Trinkaus shows, there’s more than one way to read Neanderthal trauma. But the small numbers of injured bones left in the fossil record make it hard to know which interpretation is correct.