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An artist's depiction of Afrotarsius (upper left) and other early primates from Africa. Illustration by Mark A. Klingler/Carnegie Museum of Natural History, Pittsburgh, Pa.

This week, I’m going to consider origin stories that go deeper into primate history than questions of when Homo sapiens evolved or when two-legged apes, or hominids, emerged.

Today, let’s go really far back, to a time some 40 million years ago known as the Eocene. Monkeys and apes weren’t even around yet, although their common ancestor was. But where? The discovery of a new species of Eocene primate is helping address that question.

Until about 20 years ago, the answer seemed obvious: Africa. That’s where the earliest fossil evidence was found, mainly from Egypt’s Fayum Depression. Starting in the 1990s, however, relevant fossils started popping up in Asia. Paleoanthropologists now consider a 45-million-year-old primate discovered in China, called Eosimias, to be the earliest anthropoid, the group of primates that includes monkeys, apes and humans. Eosimias was tiny, weighing less than half a pound. But it possessed certain dental and jaw characteristics that link it to living anthropoids.

The newly discovered species, named Afrasia dijijidae, dates to roughly 37 million years ago and was found in Myanmar. So far, all that’s known of Afrasia is based on four isolated teeth. But the nooks, crannies, crests and bumps on those teeth reveal a few things about where the ancestors of today’s monkeys and apes came from.

The species’ teeth are similar to those of the older Eosimias and other Asian species closely related to Eosimias. But the teeth’s size and shape are almost identical to those of a North African primate that lived at about the same time as Afrasia, approximately 38 million to 39 million years ago. It’s name is Afrotarsius. The findings are reported today by Jean-Jacques Jaeger of the University of Poitiers in France and colleagues in the Proceedings of the National Academy of Sciences.

The team suggests that the similarity in age between Afrasia and Afrotarsius indicates that a lineage, or lineages, of Asian anthropoids must have arrived in Africa only shortly before the appearance of Afrotarsius. If anthropoids had gotten to Africa much earlier, then Africa’s anthropoids would have evolved in their own direction, and millions of years later, you wouldn’t expect anthropoids in Asia and Africa to be so similar.

To get to Africa, anthropoids had to find a way across the Tethys Sea. The Tethys was a more sprawling version of the Mediterranean, drowning parts of northeastern Africa, the Middle East and West Asia. The small primates likely rafted over on giant mats of floating vegetation. Rafting may sound far-fetched, but researchers have suggested it’s how a variety of animals reached new land masses in the past. And around the same time that the ancestors of monkeys and apes left Asia for Africa, it appears some rodents did, too.

Come back on Wednesday for a look at the surprisingly European origins of the ancestor of Africa’s apes: chimpanzees, gorillas and humans.

This skull from the Petralona Cave is one of the few hominid fossils found in Greece that date to the Middle Pleistocene. Image courtesy of Wikicommons

Greece should be filled with hominid bones and stone tools. Its location makes it the perfect gateway to Europe for the earliest hominids leaving Africa, and even during dry and cold spells that made many other parts of the world uninhabitable, Greece remained pleasant. Yet the country’s archaeological record is bare from 1.8 million to 125,000 years ago, a period known as the Early to Middle Pleistocene.

And here’s why: Only 2 to 5 percent of Greece’s paleoanthropological record from this period has survived. That’s the conclusion of the authors of a new study in Quaternary Science Reviews that looks at the geological processes that preserve or destroy bones and artifacts.

To be fair, Greece’s record isn’t completely empty. Anthropologists have found some handaxes and a few skulls, a Homo heidelbergensis in the cave of Petralona and two Neanderthals in Apidima. The problem is that these finds are poorly dated. Many of the discoveries have been made on the surface, meaning there’s no geological context or stratigraphy—the depositional layers that build up in a sequence over time—to help researchers figure out when the fossils and tools were left behind. And without dates, these pieces of evidence are hard to interpret.

One explanation for the lack of discoveries is that hominids never really set down roots in the area. If they didn’t live there, there was nothing to leave behind. Vangelis Tourloukis of the University of Tübingen in Germany and Panagiotis Karkanas of Ephoreia of Palaeoanthropology–Speleology of Southern Greece don’t buy this explanation. So they looked to the  region’s geology to solve the puzzle of the missing hominids, reviewing a range of previous studies.

One thing they considered was the changing sea level over time. During cold periods, more of the world’s water is locked in polar ice sheets and glaciers, and sea level recedes, exposing parts of the seafloor. When it gets warm again, the ice melts and the ocean rises. Tourloukis and Karkanas found that during parts of the Early and Middle Pleistocene, much of the Aegean Sea, east of Greece, was dry land. In fact, the total area that was exposed then equals the area of the Greek Peninsula today (more than 50,000 square miles). If you assume all dry land was a possible living site of hominids, that means half of the potential archaeological record is now gone, submerged beneath the Aegean, the researchers say.

Back on dry land, a range of climatic and geologic factors influenced the likelihood that bones and artifacts were preserved. One of the biggest contributors was water: Rivers and streams eroded the landscape, washing sediments (and artifacts) away and piling them up somewhere else. In the Early and Middle Pleistocene, climatic conditions led to periodic catastrophic flooding, the researchers noted, and “archaeological assemblages [were] subjected to disturbance, reworking or total destruction every few thousands, hundreds or even tens of years.”

Tectonic activity, the movement within Earth’s crust and mantle that shapes topography, caused further problems. Greece is a very tectonically active region, and in the Early and Middle Pleistocene, the crust was being stretched. At one point, the stretching changed directions, raising blocks of earth and exposing bones and artifacts to destructive erosion for thousands of years. (Meanwhile, some blocks were buried, which helped protect artifacts. Such basins are probably where most potential archaeological sites are today.)

Another issue is Greece’s rugged, steep terrain. More than half of the country is mountainous or hilly, where landslides can easily bury or destroy archaeological sites.

After reviewing this geological evidence, the pair’s final step was to estimate how much of Greece’s archaeological record from this period may still exist. This takes a little bit of math. Here are the important numbers:

10 percent: Not all of Greece’s land is composed of Early to Middle Pleistocene-aged deposits. Sediments from other time periods also make up the landscape. The researchers estimated about 10 percent of the Greek Peninsula is dated to this period.

40 percent: This is the area of Greece that isn’t too steep and mountainous for fossils and tools to be preserved over time.

50 percent: Right off the bat, the researchers eliminated half of the potential archaeological record because it’s now at the bottom of the Aegean Sea.

So, the amount of the potential archaeological record that may still be out there is 10 percent of the 40 percent of the 50 percent—or just 2 percent. With some tweaks in their expectations and assumptions, the researchers say it could be as high as 5 percent.

These odds don’t seem great, but Tourloukis and Karkanas have an optimistic outlook. Because so much more land was exposed in the past, forming a natural land bridge with Turkey, hominids dispersing from North Africa through the Sinai Peninsula and the Middle East could have easily followed the southern coast of Turkey into coastal Greece and then on to Italy and the rest of Europe. And the geological evidence suggests the landscape would have been home to numerous lakes, lagoons, marshes and streams rich in valuable plant and animal resources. Why wouldn’t hominids have wanted to live there?

With this new assessment, archaeologists now have a better chance of finding traces of these hominid Shangri-Las.

What's going on in this guy's head? Read How to Think Like a Neanderthal to find out. Image courtesy of Wikicommons

Summer is just around the corner, and it’s the perfect time to start thinking about what books to bring to the beach. If you’re looking for something more cerebral than the latest romance novel or courtroom thriller, consider one of the latest books covering human evolution. Here are a few picks.

Lone Survivors: How We Came to Be the Only Humans on Earth. Paleoanthropologist Chris Stringer of the Natural History Museum in London has been one of the biggest proponents of a theory of modern human origins called Out of Africa, which suggests that Homo sapiens evolved in Africa about 200,000 years ago, left the continent and then completely replaced all other hominids without any interbreeding. Recent findings—such as the sequencing of the Neanderthal genome and the discovery of Neanderthal-human matings—have shown that the story of our species’ origin and dispersal around the world was more complicated than Stringer had realized. In Lone Survivors, he discusses the latest fossil and genetic evidence and provides an updated look at  how our species came to be. (If you’re looking for an abridged version, Stringer discusses his new thinking at Edge.)

Masters of the Planet: The Search for Our Human Origins. If you’re looking for a broader survey of human evolution, try Ian Tattersall’s Masters of the Planet. Tattersall, a curator emeritus at the American Museum of Natural History in New York, covers the last seven million years of hominid history, starting with the cast of contenders for earliest hominid. He ends by discussing how our superior brains and capacity for symbolic thinking and language allowed us to become masters of the planet, so to speak.

The Fossil Chronicles: How Two Controversial Discoveries Changed Our View of Human Evolution. Two of my favorite subjects in human evolution—the Hobbit (a.k.a. Homo floresiensis) and the Taung Child—are the subject of Dean Falk’s The Fossil Chronicles. On the face of it, the two don’t have much in common. But Falk, an anthropologist at Florida State University who studies brain evolution, argues that both fossils changed the course of human evolution studies. She recounts the history of the discoveries and traces how they changed scientific thinking: The Hobbit showed that humans shared the world with other hominids until very recently, while the Taung Child forced anthropologists to realize Africa was the birthplace of our lineage.

How to Think Like a Neanderthal. One of the most enduring debates in human evolution centers on Neanderthal intellect. Were they smart, or were they dumb? In How to Think Like a Neanderthal, anthropologist Thomas Wynn and psychologist Frederick Coolidge, both of the University of Colorado in Colorado Springs, try and get into the Neanderthal mind by examining the genetic, fossil and archaeological records.

Across Atlantic Ice: The Origin of America’s Clovis Culture. Everyone supposedly knows how people came to the New World. About 12,000 years ago, the ancestors of the first Americans left Asia and walked across a land bridge spanning the Bering Strait. But archaeologists Dennis Stanford of the Smithsonian’s National Museum of Natural History and Bruce Bradley of the University of Exeter in England provide a controversial alternative scenario in Across Atlantic Ice. They argue that Europeans aboard boats may have gotten here first, about 18,000 years ago, by following the edges of an ancient ice sheet across the Atlantic.

An oldie but a goodie, Lucy: The Beginnings of Humankind. Lucy is not new, but it’s still a great read that offers an insider’s view on how paleoanthropology works. Written by Lucy’s discoverer, Don Johanson, in 1981, the book gives readers a firsthand look at how the iconic fossil was discovered and how researchers went about studying her. If you want a more contemporary look at Lucy, try Johanson’s 2010 book, Lucy’s Legacy.

Another classic, In the Shadow of Man. Now I’ve broken two rules. In the Shadow of Man is not new nor is it really about human evolution. It’s Jane Goodall’s first book. Written in the early 1970s, it chronicles the beginning of Goodall’s decades-long study of the chimpanzees at Gombe National Park in Tanzania and highlights some of her early discoveries, including the chimps’ use of tools. If you’re a wildlife fan, this is a great treat.

An artist's conception of the unusual humans living in southwestern China 11,500 to 14,300 years ago. Image © Peter Schouten

Last week, an international group of researchers reported the discovery of fossils belonging to a strange population of hominids that lived in southwestern China as recently as 11,500 years ago, at the end of the Pleistocene Epoch. The fossils resemble modern humans in many ways but possess some unusual characteristics. The traits may be evidence that Homo sapiens were more diverse in the past—or a sign that scientists have uncovered a new species.

Anthropologist Darren Curnoe of the University of New South Wales in Australia led the analysis of the fossils, detailed in the journal PLoS ONE. The bones—a partial skull, skull cap, jaws and teeth—came from Longlin Cave in Guangxi Province and Malu Cave in Yunnan Province, and date to  11,500 to 14,300 years ago. In comparing the Chinese bones with those of recent humans, H. sapiens living during the Pleistocene, Neanderthals and Homo erectus, the researchers concluded the Chinese fossils have a unique mix of modern features and traits rarely, if ever, seen in recent and Pleistocene humans, such as a very broad face and a protruding jaw.

The most dramatic interpretation of the fossils is that they represent a newly discovered species that lived alongside modern humans in East Asia until very recently. Anthropologist Chris Stringer of the Natural History Museum in London thinks that’s a feasible idea. In fact, the fossils could be the remains of the mysterious Denisovans, Stringer told New Scientist. Scientists discovered the Denisovans a few years ago while analyzing DNA recovered from a finger bone found in a Siberian cave that dated to 30,000 to 48,oo0 years ago. The DNA didn’t align with that of modern humans or Neanderthals, the only species known to inhabit the area at the time. Since then, scientists have been looking to match a face to the DNA. This idea will be confirmed only if the researchers manage to retrieve DNA from any of the Chinese fossils.

A less headline-worthy explanation is that these hominids were members of an early, unknown migration of H. sapiens out of Africa. (Genetic evidence indicates there were at least two migrations into Eurasia: one at 60,000 to 70,000 years ago and another at 30,000 to 40,000 years ago.) Once these people settled in East Asia, they somehow remained isolated from other human populations for thousands of years and eventually died out without leaving behind descendants. Under this scenario, the population’s unusual features suggest our species was more diverse thousands of years ago than it is today. This possibility is supported by other fossils found in Africa. Curnoe and his colleagues describe H. sapiens fossils found in East, South and North Africa, dating from 12,000 to 100,000 years ago, that possess a mix of modern and more primitive traits.

This situation reminds me of the search for the earliest modern humans. Many of these fossils also retained primitive features, which has made it difficult for anthropologists to decide which ones are truly modern humans and which ones aren’t. Likewise, anthropologists now have to determine whether modern humans could have been more diverse near the end of the Pleistocene than they had previously thought or whether more hominid species were living back then than they had previously expected.

The skull of Australopithecus sediba. Picture by Brett Eloff, courtesy of Lee Berger and the University of the Witwatersrand

For this last Hominid Hunting post of 2011, I reviewed recent human evolution research highlights to come up with my picks for the top 10 hominid discoveries of the year. While genetic breakthroughs have hogged the spotlight the past couple of years, good old-fashioned fossil and archaeological finds were front and center in 2011.

10. Earliest Modern Humans in Europe: Paleoanthropologists believe modern humans (Homo sapiens) came to Europe about 43,000 years ago. This date is based on the age of sophisticated stone tools, not human fossils. This year two teams dated European fossils that are in line with the age of the tools: A human upper jaw discovered in southern England in 1927 was dated to  44,000 years ago, and two molars unearthed in Italy were dated to 45,000 years ago. These fossils are the oldest known human remains on the continent.

9. The Arches of Australopithecus afarensis: There’s no doubt that Lucy and her species, Australopithecus afarensis, walked upright. But the degree to which these hominids walked on the ground has been debated. The discovery of a 3.2-million-year-old foot bone confirmed that Lucy and her kind had arched feet and therefore probably walked much like modern people. The researchers who studied the fossil say it indicates Australopithecus afarensis no longer needed to spend much time in the treetops; however, other researchers disagree, saying hominids at this time were still good tree climbers.

8. World’s Earliest Mattress: In a rock shelter in South Africa, archaeologists uncovered a 77,000-year-old mattress composed of thin layers of sedges and grasses, predating all other known mattresses by 50,000 years. Early humans knew how to keep the bed bugs out; the bedding was stuffed with leaves from the Cape Laurel tree (Cryptocarya woodii), which release chemicals known to kill mosquitos and other bugs.

7. Neanderthal Mountaineers: Neanderthals evolved many traits to deal with the cold; for example, their short limbs helped them conserve heat. A mathematical analysis revealed that short limbs may have also helped Neanderthals walk more efficiently in mountainous terrains. Specifically, the fact that Neanderthals had shorter shins relative to their thighs meant they didn’t need to lift their legs as high while walking uphill, compared to modern people with longer legs. “For a given step length, they [needed] to put in less effort,” said lead research Ryan Higgins of Johns Hopkins University.

6. The First Art Studio: Archaeologists working in South Africa’s Blombos Cave discovered early humans had a knack for chemistry. In a 100,000-year-old workshop, they found all of the raw materials needed to make paint, as well as abalone shells used as storage containers—evidence that our ancestors were capable of long-term planning at this time.

5. Australopithecine Females Strayed, Males Stayed Close to Home: In many monkey species, when males reach adolescence, they leave their home to search for a new group, probably as a way to avoid breeding with their female relatives. In chimpanzees and some humans, the opposite occurs: Females move away. Now it appears that australopithecines followed the chimp/human pattern. Researchers studied the composition of strontium isotopes found in the teeth of members of Australopithecus africanus and Paranthropus robustus. An individual consumes strontium through food and it is taken up by the teeth during childhood. Because the isotopes (different forms of the element) in plants and animals vary by geology and location, strontium can be used as a proxy for an individual’s location before adulthood. In the study, the researchers discovered that large individuals, presumably males, tended to have strontium isotope ratios typical of the area where the fossils were found; smaller individuals, or females, had non-local strontium isotope ratios, indicating they had moved into the area as adults.

4. Confirmation of Pre-Clovis People in North America: Since the 1930s, archaeologists have thought the Clovis people, known for their fluted projectile points, were the first people to arrive in the New World, about 13,000 years ago. But in recent years there have been hints that someone else got to North America first. The discovery of more than 15,000 stone artifacts in central Texas, dating to between 13,200 and 15,500 years ago, confirmed those suspicions. Corroborating evidence came from Washington State, where a mastodon rib containing a projectile point was dated this year to 13,800 years ago.

3. Denisovans Left A Mark in Modern DNA: The Denisovans lived in Eurasia sometime between 30,000 and 50,000 years ago. Scientists don’t know what they looked like; the only evidence of this extinct hominid group is DNA extracted from a bone fragment retrieved from a cave in Siberia. But this year, several studies revealed the mysterious population bred with several lineages of modern humans; people native to Southeast Asia, Australia, Melanesia, Polynesia and elsewhere in Oceania carry Denisovan DNA.

2. Out of Africa and Into Arabia: Traditionally, paleoanthropologists have thought modern humans left Africa through the Sinai Peninsula and into the Levant. But some researchers suggest our ancestors took a more southerly route, across the Red Sea and into southern Arabia. This year, several studies provided evidence pointing to this exit strategy. First, a team reported the discovery of 125,000-year-old stone tools in the United Arab Emirates. The researchers suggested humans ventured into Arabia when sea level was lower, making a trip across the Red Sea easier. (Geologists later verified the climate would have been just right at this time.) No fossils were found with the tools, but the scientists concluded they belonged to modern humans rather than Neanderthals or some other contemporaneous hominid. Another study this year complemented the finding: Paleoanthropologists also found stone tools, dating to 106,000 years ago, in Oman. The researchers say the artifacts match tools of the Nubian Complex, which are found only in the Horn of Africa. This connection implies the makers of those African tools, most likely modern humans, made the migration into Oman.

1. Australopithecus sediba, Candidate for Homo Ancestor: Last year, scientists announced the discovery of a new hominid species from South Africa’s Cradle of Humankind—Australopithecus sediba. This year, the researchers announced the results of an in-depth analysis of the 1.97-million-year-old species. They say a mix of australopithecine and Homo-like traits make Australopithecus sediba, or a species very similar to it, a possible direct ancestor of our own genus, Homo.