October 3, 2012
Archaeologists have something new to add to the record books: the earliest case of anemia. Two 1.5-million-year-old skull fragments unearthed in Tanzania display tell-tale signatures of the blood disorder—and may offer hints on the meat-eating habits of our ancestors.
The fossil pieces come from Olduvai Gorge and belong to an approximately 2-year-old child. The fragments are not enough to identify the toddler’s species, but based on the age and location, Homo erectus is a good possibility. On certain portions of the fossils Manuel Domínguez-Rodrigo of Madrid’s Complutense University and colleagues noticed the bone was extremely porous. After ruling out several possible causes of the damage, the team concluded the individual had suffered from porotic hyperostosis. This condition causes the outer cranial bone to thin and exposes the spongy inner bone, which starts to grow abnormally. This is the first time porotic hyperostosis has been seen in a hominid from the early Pleistocene, the team reports in PLOS ONE.
Porotic hyperostosis can be a manifestation of anemia, which is caused by a decline in oxygen-carrying red blood cells. The researchers say the most common cause of the anemia that leads to porotic hyperostosis in children is a lack of vitamins B12 and B6 (with parasites and gastrointestinal infections contributing to the disorder). The nutritional deficiency probably occurred either because the child was still nursing and his/her mother lacked the B vitamins herself or the child was being weaned and was not yet getting adequate levels of the vitamins in his/her own food.
How does this relate to eating meat?
Domínguez-Rodrigo and his colleagues suggest the insufficient levels of B12 and B6 were ultimately the result of not eating enough meat, which is rich in those vitamins. The researchers argue that by 1.5 million years ago hominid physiology had become so dependent on meat that not ingesting proper amounts of it led to nutritional deficiencies. (In contrast, anemia-induced porotic hyperostosis is almost never seen in chimpanzees, which consume much smaller amounts of animal protein.) Thus, the researchers conclude, this early case of anemia is one more piece of evidence that meat-eating was a crucial part of the hominid diet by the early Pleistocene.
September 5, 2012
The earliest known instance of cannibalism among hominids occurred roughly 800,000 years ago. The victims, mainly children, may have been eaten as part of a strategy to defend territories against neighbors, researchers report online in the Journal of Human Evolution. The new study shows how anthropologists use the behavior of modern humans and primates to make inferences about what hominids did in the past—and demonstrates the limitations of such comparisons.
The cannibalism in question was discovered in the Gran Dolina cave site of Spain’s Atapuerca Mountains. Eudald Carbonell of the University of Rovira and Virgili in Spain and colleagues found evidence of butchering on bones belonging to Homo antecessor, a controversial species that lived in Europe as early as 1.2 million years ago. Because no other hominid species has been found in the region at the same time as the butchered bones, the victims must have been eaten by their own kind, the team concluded in 2010 in the journal Current Anthropology (PDF).
Today, human cannibalism occurs in a variety of contexts: for nutritional value (often in times of starvation), as part of funerary rituals or during warfare. The different purposes of cannibalism can leave different patterns in the archaeological record. When humans consume other humans for purely dietary reasons, the victims are often treated just like any other prey. This is what the researchers found at Gran Dolina. Eleven individuals were butchered in a manner similar to that of deer and other mammals: Bones had cut marks in areas of muscle attachments and the skulls had signs of defleshing. Thus, H. antecessor appeared to eat its own kind for a nutritional purpose—but probably not because of a food shortage, as the team says there’s evidence of cannibalism over an extended period of time, dozens or even hundreds of years.
So why cannibalism? To find an answer, the researchers looked to chimpanzees. That’s because some aspects of H. antecessor cannibalism don’t resemble those of contemporary human cannibalism or cannibalism seen in Neanderthals or early modern humans living 100,000 years ago. For instance, nine of the 11 butchered individuals at Gran Dolina were children or adolescents compared with the largely adult victims of more recent human cannibalism.
Young victims is a pattern seen among chimpanzees. When female chimps range alone near the boundary of their territory, males from the neighboring group may kill and eat the females’ infants. Carbonell and his colleagues suggest the best explanation for this behavior is territorial defense and expansion. Males may attack to scare off other chimps as a way to protect their resources and gain new land to roam; such attacks are easiest against vulnerable females and their young, which make good meals. The team likewise concludes a similar explanation may have been the motivation behind H. antecessor cannibalism.
Whether this is a reasonable conclusion depends on some unanswered questions. For example, the researchers assume that the cannibalism was the result of intergroup violence and aggression, but they offer no evidence that the H. antecessor cannibals came from a different group than the victims. If they were all members of the same clan, then territorial defense doesn’t seem likely. It also seems unlikely if H. antecessor‘s social structure was vastly different from chimps—in which groups of probably related males band together to actively defend a territory while females in a community often forage alone with their infants.
It looks like the team has some more work to do.
August 29, 2012
Have you ever wondered why women stay pregnant for nine months? For decades, anthropologists have explained the timing of human gestation and birth as a balance between two constraints: the size of a women’s hips and the size of a newborn’s brain. But new research says that’s not the case. Instead, the timing of childbirth occurs when women’s bodies can no longer keep up with the energy demands of pregnancy. That happens at around nine months, Holly Dunsworth of the University of Rhode Island and colleagues report online August 27 in the Proceedings of the National Academy of Sciences.
The traditional explanation of gestation length is known as the obstetric dilemma. The hypothesis suggests that the width of the pelvis, and thus the width of the birth canal, is limited by the demands of efficient upright walking. But as brain size expanded over hominid evolution, heads got bigger. To make sure a baby’s head could fit through the birth canal, gestation decreased and babies were born at an earlier stage of development; today, newborns enter the world with the least developed brain of all primates at less than 30 percent adult size.
Dunsworth and her colleagues wanted to see if they could find any actual evidence to support the obstetric dilemma. First, they considered gestation length. Traditionally, human gestation has been considered short when looking at how much additional growth the brain needs to reach adult size. But such a measure is unfair when compared to other primates since humans have abnormally large brains, the researchers say. Instead, Dunsworth’s team compared gestation length to maternal body size and found humans actually have relatively long pregnancies—37 days longer than would be expected for a typical primate our size. Our gestation is also relatively extended compared with chimpanzees or gorillas, suggesting pregnancies got longer, not shorter, in hominids.
The team also looked for evidence that widening the pelvis to accommodate bigger brained babies would make walking less efficient. Researchers have assumed that broadening the hips would increase the force needed by hip muscles to walk and run, thus making locomotion less energy efficient. But one recent study shows the dimensions of the hips don’t actually affect the muscle’s required force, calling into question the long-held belief that wider hips would interfere with women’s walking. Furthermore, the team calculated how much wider the hips would have to be if humans were born with the same brain development as chimps (40 percent adult size). All that would be needed is a three-centimeter increase. Women’s hips already vary by three or more centimeters, the researchers say, suggesting that hip size really doesn’t limit gestation.
Instead, gestation is determined by energy. Studies of mammals show that during pregnancy females reach their species’ “metabolic ceiling,” the upper limit of the amount of energy they can expend. In humans, the metabolic ceiling is 2 to 2.5 times the baseline amount of energy needed during rest. Dunsworth and her colleagues say women reach that limit by their sixth month of pregnancy. Then at nine months, the energy demands of a fetus go beyond this metabolic threshold. “Extending gestation even by a month would likely require metabolic investment beyond the mother’s capacity,” the team writes.
But even though hip size doesn’t appear to limit the size of a baby’s head, women around the world often have trouble delivering babies because of the tight fit of the head going through the birth canal. One possible explanation is that childbirth has only become problematic recently in human evolution. Changes in diet that have led to increased energy consumption may be allowing women to produce bigger babies, and natural selection hasn’t had enough time to broaden the hips. Figuring out why modern childbirth is so difficult, and dangerous, is an area that needs further research.
August 13, 2012
Paranthropus and Homo both emerged in South Africa roughly 1.8 million years ago and lived side by side for several hundred thousand years. Differences in their diet have been used to explain why the Homo lineage succeeded while Paranthropus died out. Now, new chemical analyses of fossil teeth further confirm that the two hominids dined on different foods, with Homo eating more meat than Paranthropus. But even with these differences, the two genera appeared to have ranged over the South African landscape in similar ways.
They can look at the size and shape of the teeth, jaw and skull and look at the diet of modern animals with similar characteristics. They can also observe the microscopic scratches and pits on a tooth’s chewing surface to determine how hard or abrasive one’s diet was. A third option is to investigate the chemistry of an individual’s teeth and bones, which is derived from the chemistry of what the animal ate.
Vincent Balter of Ecole Normale Supérieure de Lyon in France and colleagues selected the third method for their research, published last week in Nature. They analyzed the dental chemistry of seven Paranthropus robustus specimens, three early Homo specimens (species not known) and four members of Australopithecus africanus, which lived lived in South Africa 3.3 million to 2.1 million years ago. All of the teeth came from the famous cave sites of Sterkfontein, Swartkrans and Kromdraai.
Using a laser, the team removed tiny amounts of the dental enamel to measure strontium, barium and calcium isotopes. (Isotopes of an element have different numbers of neutrons.) The ratio of these isotopes tend to change as you go up the food chain. Low barium-to-calcium or strontium-to-calcium ratios, for example, are typical of carnivores. John Hawks has a good explanation of how anthropologists use such ratios to examine diet on his blog.
Looking at these isotope ratios, a clear pattern emerged. Meat was a large component of Homo‘s diet whereas plants were a big part of P. robustus‘ diet. These results are in line with previous studies. A. africanus ate both types of food. The researchers speculate the species probably ate a lot of “woody” plants (fruits and leaves, not grasses) during certain seasons and meat during other times of the year, although they can’t say which foods were eaten during which seasons. Taken together, these results suggest earlier hominids were generalists, and then around two million years ago, they began to specialize more. The addition of meat in Homo‘s diet may have allowed our ancestors to evolve big brains, which require a lot of energy to support.
The team also looked at a third isotope ratio, strontium-87 to strontium-86. Strontium isotopes vary by the geology of the local bedrock, so variations in this isotope ratio indicate hominids were eating foods in different locations. These ratios were pretty much the same for all three hominid species, suggesting they all had similar home ranges. So even though Paranthropus and Homo had different diets, they traveled around in similar areas and traversed similar amounts of territory.
To get an even better look at how diets changed with the origin of Homo and Paranthropus, Balter and his colleagues suggest similar tests should be conducted on the teeth of Australopithecus sediba—the 1.97-milion-year-old species that some anthropologists say is a candidate for the ancestor of Homo.
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.