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.
August 6, 2012
Welcome to Hominid Hunting’s new series “Becoming Human,” which will periodically examine the evolution of the major traits and behaviors that define humans, such as big brains, language, technology and art. Today, we look at the most fundamental human characteristic: walking upright.
Walking upright on two legs is the trait that defines the hominid lineage: Bipedalism separated the first hominids from the rest of the four-legged apes. It took a while for anthropologists to realize this. At the turn of the 20th century, scientists thought that big brains made hominids unique. This was a reasonable conclusion since the only known hominid fossils were of brainy species–Neanderthals and Homo erectus.
That thinking began to change in the 1920s when anatomist Raymond Dart discovered the skull known as the Taung Child in South Africa. Taung Child had a small brain, and many researchers thought the approximately three-million-year-old Taung was merely an ape. But one feature stood out as being human-like. The foramen magnum, the hole through which the spinal cord leaves the head, was positioned further forward under the skull than an ape’s, indicating that Taung held its head erect and therefore likely walked upright. In the 1930s and 1940s, further fossil discoveries of bipedal apes that predated Neanderthals and H. erectus (collectively called australopithecines) helped convince anthropologists that walking upright came before big brains in the evolution of humans. This was demonstrated most impressively in 1974 with the finding of Lucy, a nearly complete australopithecine skeleton. Although Lucy was small, she had the anatomy of a biped, including a broad pelvis and thigh bones that angled in toward the knees, which brings the feet in line with the body’s center of gravity and creates stability while walking.
In more recent decades, anthropologists have determined that bipedalism has very ancient roots. In 2001, a group of French paleoanthropologists unearthed the seven-million-year-old Sahelanthropus tchadensis in Chad. Known only from a skull and teeth, Sahelanthropus‘ status as an upright walker is based solely on the placement of its foramen magnum, and many anthropologists remain skeptical about the species’ form of locomotion. In 2000, paleoanthropologists working in Kenya found the teeth and two thigh bones of the six-million-year-old Orrorin tugenensis. The shape of the thigh bones confirms Orrorin was bipedal. The earliest hominid with the most extensive evidence for bipedalism is the 4.4-million-year-old Ardipithecus ramidus. In 2009, researchers announced the results of more than 15 years of analysis of the species and introduced the world to a nearly complete skeleton called Ardi.
Although the earliest hominids were capable of upright walking, they probably didn’t get around exactly as we do today. They retained primitive features—such as long, curved fingers and toes as well as longer arms and shorter legs—that indicate they spent time in trees. It’s not until the emergence of H. erectus 1.89 million years ago that hominids grew tall, evolved long legs and became completely terrestrial creatures.
While the timeline of the evolution of upright walking is well understood, why hominids took their first bipedal steps is not. In 1871, Charles Darwin offered an explanation in his book The Descent of Man: Hominids needed to walk on two legs to free up their hands. He wrote that “…the hands and arms could hardly have become perfect enough to have manufactured weapons, or to have hurled stones and spears with a true aim, as long as they were habitually used for locomotion.” One problem with this idea is that the earliest stone tools don’t show up in the archaeological record until roughly 2.5 million years ago, about 4.5 million years after bipedalism’s origin.
But after the unveiling of Ardi in 2009, anthropologist C. Owen Lovejoy of Kent State University revived Darwin’s explanation by tying bipedalism to the origin of monogamy. I wrote about Lovejoy’s hypothesis for EARTH magazine in 2010. Lovejoy begins by noting that Ardi’s discoverers say the species lived in a forest. As climatic changes made African forests more seasonal and variable environments, it would have become harder and more time-consuming for individuals to find food. This would have been especially difficult for females raising offspring. At this point, Lovejoy suggests, a mutually beneficial arrangement evolved: Males gathered food for females and their young and in return females mated exclusively with their providers. To be successful providers, males needed their arms and hands free to carry food, and thus bipedalism evolved. This scenario, as with all bipedalism hypotheses, is really hard to test. But earlier this year, researchers offered some support when they found that chimpanzees tend to walk bipedally when carrying rare or valuable foods.
Another theory considers the efficiency of upright walking. In the 1980s, Peter Rodman and Henry McHenry, both at the University of California, Davis, suggested that hominids evolved to walk upright in response to climate change. As forests shrank, hominid ancestors found themselves descending from the trees to walk across stretches of grassland that separated forest patches. The most energetically efficient way to walk on the ground was bipedally, Rodman and McHenry argued. (Full disclosure: Rodman was my graduate school advisor.) In 2007, researchers studying chimpanzees on treadmills determined that the chimps required 75 percent more energy while walking than two-legged humans, providing some evidence that bipedalism has advantages.
Numerous other explanations for bipedalism have been outright rejected, such as the idea that our ancestors needed to stand up to see over tall grass or to minimize the amount of the body exposed to the sun in a treeless savannah. Both ideas were debunked by the fact that the first hominids lived in at least partially wooded habitats.
Although difficult to study, the question of why bipedalism evolved might come closer to an answer if paleoanthropologists dig up more fossils of the earliest hominids that lived seven million to six million years ago. Who knows how many species of bipedal apes they’ll find. But each new discovery has the potential to fundamentally change how we understand the origins of one of our most distinctive traits.
June 20, 2012
Call someone a baboon, and you might have to prepare for a fight. But if you called Homo erectus a baboon—and if one were alive today—he or she might say, “Yep.”
That’s because H. erectus probably lived in complex, multilevel societies similar to those of modern hamadryas baboons. At least, that’s the case anthropologists Larissa Swedell and Thomas Plummer, both at Queens College, City University of New York, make in the International Journal of Primatology. Swedell and Plummer argue that a dry environment led both species to evolve intricate social structures.
Hamadryas baboons live in the semidesert lowlands of the Horn of Africa and the southwestern portion of the Arabian Peninsula. Their social lives are organized in a four-tiered system. A few hundred of the monkeys aggregate in a large, loose group called a troop. Troops huddle together on their sleeping cliffs at night to deter predators. During the day, troops splinter into smaller groups because it’s a more efficient way to forage in a desert, where food tends to be sparse and spread out, especially during certain seasons. The smallest group is the one-male unit: one adult male, one or more adult females and their young offspring. Several of these units form a clan. When it’s time for a young male to find his own unit, he typically stays within his clan. Because males stay close to home, neighboring males tend to be relatives and therefore cooperate with each other—even tolerating the “kidnapping” of their females by their brethren. Finally, several clans make up a band, which travels over a common home range.
H. erectus evolved 1.9 million years ago. Swedell and Plummer note that climatic changes that occurred 2.8 million years ago, 1.7 million years ago and 1 million years ago created a drier and more variable environment for the species than what any previous hominid had experienced. H. erectus lived in more open habitats and had to travel greater distances to find food. Like hamadryas baboons, this probably favored smaller foraging groups during the day and larger communities at night for safety.
As H. erectus traveled more and dealt with new habitats, it added new food to its diet: meat and underground tubers. Getting both required new technologies. The greater cognitive demands of such procurement may in part explain why the species evolved larger brains. This created some challenges for females, however. Big brains require a lot of energy. As a consequence, Swedell and Plummer suggest, feeding and raising bigger-brained babies may have been too big a task for a female to accomplish on her own.
To help one another raise offspring, females may have started to live in small groups with their female kin. (Postmenopausal grandmothers may have been especially useful in helping to rear their grandchildren.) Selection would have favored males who could monopolize such groups. In exchange for exclusive breeding rights, males could help females protect and perhaps even feed their children. The result: a group analogous to the hamadryas baboon’s one-male units. The benefits of male cooperation in defending groups from outside males or predators may have led to the formation of larger groups akin to the baboon’s clans and bands. Clan (or band) males may have also worked together to hunt large game.
H. erectus societies weren’t carbon copies of baboon groups. Female hamadryas baboons, for example, don’t help each other raise offspring, and females within a one-male unit aren’t typically related. But the baboon analogy allows anthropologists to include two social phenomenon that are usually considered mutually exclusive in many models of hominid social evolution, Swedell and Plummer say. In these multilevel societies females bonded with both males (and not in monogamous pairs) and one another.
The hamadryas baboon model is more than a good story, Swedell and Plummer say. More research into the baboon’s social lives could help bolster the team’s theories on why the monkeys live in multilevel societies, and more archaeological work could further support the ideas that H. erectus traveled far distances, lived in more open habitats and ate foods that were more difficult to collect.
May 23, 2012
Iraq is the home of the Fertile Crescent, the Cradle of Civilization. But the country’s importance in human history goes back even further, to the time of the Neanderthals. In 1951, American archaeologist Ralph Solecki discovered Neanderthal remains in Shanidar Cave. The cave sits in the Zagros Mountains in the Kurdistan region of northern Iraq, about 250 miles north of Baghdad. From 1951 to 1960, Solecki and colleagues excavated the cave and recovered fossils belonging to 10 individuals dating to between 65,000 and 35,000 years ago. Politics prevented further archaeological work, but the Shanidar fossils still provide important insights on the Neanderthals of West Asia. Here are a few of the most intriguing finds:
Shanidar 1: Nicknamed Nandy, Shanidar 1 lived sometime between 45,000 and 35,000 years ago. He had a hard life. A blow to the head in his youth probably blinded him in his left eye. A withered right arm and leg suggest the head injury probably also caused brain damage that paralyzed the right side of Nandy’s body. He also fractured his foot at some point. Yet his bones all show signs of healing, and Nandy lived to be a senior citizen by Neanderthal standards, dying sometime between the ages of 35 and 45. The find revealed that Neanderthals must have taken care of their sick and wounded.
Shanidar 3: Also an adult male, Shanidar 3 had plenty of problems of his own. In addition to suffering from arthritis, the Neanderthal seems to have been violently attacked. A tiny groove on one of his ribs indicates he was probably struck in the chest. A 2009 analysis (PDF) points to a modern human, Homo sapiens, as the assailant. Based on experimental stabbings of pig carcasses, a team led by Steven Churchill of Duke University determined that the most likely weapon was some kind of dart, shot from long range. Because modern humans are the only hominids known to have made projectile weapons, the researchers blamed our species for the wound. The wound may have harmed Shandiar 3′s lungs, but it’s possible he survived the attack. A callous that formed over the groove shows that he must have lived at least a few week after the incident. And modern people with similar injuries can survive even with little medical care.
Today, you can examine Shanidar 3 for yourself at Smithsonian’s National Museum of Natural History, where the fossil is on display.
Shanidar 4: Yet another adult male, Shanidar 4 was found on his side curled up in the fetal position. An analysis of the ancient pollen found in association with the fossilized skeleton revealed bright flowers had been brought into the cave. Solecki interpreted the pollen studies as evidence that Neanderthals buried their dead and adorned the graves with flowers, suggesting Neanderthals had rituals. Skeptical anthropologists say natural forces—perhaps burrowing rodents—introduced the pretty flora into the cave. Although Neanderthals might not have decorated the graves, they were responsible for burying at least some of the individuals in Shanidar.
May 14, 2012
In Interview with the Vampire, Claudia, portrayed by Kirsten Dunst in the movie version, becomes a vampire at age 6. Six decades later, she still has the body of a child but the thoughts and desires of a grown woman.
In this way, orangutans are kind of like vampires. They have their own form of arrested development.
When male orangutans hit puberty, they develop distinct traits known as secondary sex characteristics that separate them from females. In addition to being much bigger, males grow longer, shaggier hair on their arms and back and sport giant cheek pads. They also have throat pouches that resemble large double chins, allowing males to beckon females with loud long calls.
Some males are late bloomers, not acquiring these traits until as late as age 30. But looks can be deceiving. Even though these males appear to be youngsters, they are sexually mature and capable of siring offspring.
Scientists think the two different types of adult males—those with secondary sex characteristics and those without—are two alternative mating strategies that evolved in orangutans. A new study published online in the American Journal of Physical Anthropology tries to pinpoint the circumstances under which orangutan arrested development emerges.
To do this, Gauri Pradhan of the University of South Florida and Maria van Noordwijk and Carel van Schaik, both of the University of Zurich, considered the differences between orangutans living in Borneo and those in Sumatra. These Indonesian islands are the only two places in the world where orangutans are still found in the wild. But arrested development is largely limited to Sumatra.
Orangutans in both locations are mostly solitary. They roam the treetops alone, but they live in home ranges that overlap with those of other orangutans. In Sumatra, a female prefers to mate with the dominant male that lives in her neck of the woods. This male always has his full set of male features. A female finds the dominant male by following the sound of his long call, and when she’s ready to be pregnant, the two enjoy a sort of honeymoon—traveling and mating together for up to three weeks. Other adult-looking males may live in the same area, but females actively avoid their calls and stay hidden from them.
Because the dominant male is so popular, he can be choosy about mates. These males tend to pass over inexperienced females who haven’t yet had a baby. With younger adult females, it’s hard to tell if they are truly ready to become mothers, so it’s a better bet to stick with females who are already moms.
Yet some males are interested in these naïve females: the sexually mature males lacking adult traits. Unlike the other male orangutans, these guys don’t wait for females to come to them. They search the forest for receptive females, and Pradhan and his colleagues speculate that these males might father a lot of the children of first-time orangutan moms.
The sex lives of orangutans on Borneo are quite different. Here, no single adult-looking male is dominant. Many full-fledged males mate with an area’s females. Orangutan honeymoons are much shorter, and males may fight with each over a potential mate. Because the competition is so fierce, males aren’t choosy about who they mate with—and sometimes, even if a female’s not in the mood for mating, a male might force her to copulate.
Pradhan’s team incorporated these differences, as well as some assumptions about male growth, into a mathematical model. Their equations allowed them to determine which factors best explain the presence of immature-looking adult males in a population. The most important variable, they conclude, is the ability for one male to dominate an area. When this happens, as in Sumatra, it becomes beneficial for other males to have a covert mating strategy.
But if there is a lot of direct competition among males, as in Borneo, then it’s better to be a full-fledged male, who will always beat out immature males. No one male can monopolize females in Borneo because males tend to travel more on the ground there, the researchers say. That improves their mobility and makes it easier to quickly find females, even those who may not want to be found.
Thousands of years ago, orangutans once lived throughout much of Southeast Asia, even on the mainland. I wonder how pervasive arrested development was back then. Even if we had large bone samples, would anthropologists ever be able to detect such behavior in the fossil record?