December 5, 2012
Perhaps no other human trait is as variable as human height. At 5’4″, I’d be dwarfed standing next to 6’3″ Kerri Walsh, the 2012 Olympic gold medalist in beach volleyball. But next to an African pygmy woman, I’d be a giant. The source of that variation is something that anthropologists have been trying to root out for decades. Diet, climate and environment are frequently linked to height differences across human populations.
More recently, researchers have implicated another factor: mortality rate. In a new study in the journal Current Anthropology, Andrea Bamberg Migliano and Myrtille Guillon, both of the University College London, make the case that people living in populations with low life expectancies don’t grow as tall as people living in groups with longer life spans. They also argue changes in mortality rates might account for the jump in body size from Australopithecus to Homo some 2 million years ago.
From an evolutionary standpoint, Migliano and Guillon note, it’s beneficial to start reproducing as soon as possible if you live in a society where individuals typically die young. That way you can have as many babies as possible in a short amount of time. Thus, you should stop growing relatively early in life and start devoting your energy to having children and taking care of them. Having a shorter developmental period means you can’t grow as tall, on average, as someone who has more time to mature. But getting big has reproductive benefits: Larger individuals tend to take in more energy and therefore can invest more energy in reproducing. So in societies with lower mortality rates, and longer adulthoods, it’s better to mature slowly and grow bigger and taller. Over time, populations experiencing different mortality rates will adapt to have shorter or longer developmental periods—and therefore be shorter or taller. (Of course, there is also variation within a population. But here, and throughout the post, I’m talking about population averages.)
To investigate this idea, Migliano and Guillon looked at previously collected height and mortality data from 89 small-scale populations from all over the world. These groups live in a variety of environments, including deserts, forests and savannas, and have different subsistence strategies, including hunter-gathering, pastoralism and agriculture. Using statistical analyses, the team wanted to see what kind of factors best explained the variation of heights in their data set.
In one analysis, three measures of survivorship—life expectancy at birth, life expectancy at age 15 and probability of survival to age 15—accounted for about 70 percent of height variance. The researchers also found evidence that people from societies with high mortality rates do indeed develop faster: Girls from groups that have low life expectancies start menstruating earlier than girls who are more likely to live longer. Environmental setting also influenced height, with people from savannas tending to be taller than people from forests. Diet, however, seemed to play a much smaller role, at least in the study samples.
Other variables not considered in the study may also contribute to height variation, the researchers point out. Temperature and humidity probably somehow factor in. For example, some work suggests shorter people generate less heat in hot, humid environments and therefore cool down more efficiently. That might explain why people living in tropical forests are shorter than those from savannas.
There are some situations, however, where the study’s findings don’t hold up. In modern Western societies, where mortality rates are low, growth is actually sped up because of an overabundance of food. Some studies now show that obesity may contribute to early puberty in girls. On the other hand, severe malnourishment can lead to delayed growth.
Based on the study’s findings, Migliano and Guillon suggest lower death rates probably contributed to changes in body size and height during the Australopithecus-Homo transition. In one study, anthropologists estimated early Homo species were about 30 percent bigger than australopithecines. Homo erectus grew even taller, within the range of variation of modern people. The larger brain of the genus Homo may have allowed the group to lower its mortality rate by outsmarting predators or foraging more efficiently than Australopithecus. Within H. erectus, differences in mortality rates between populations—which lived over a much larger geographic expanse than australopithecines—probably accounts for the variation of height seen in the fossil record of that species.
Much more investigation is needed to corroborate the link between death and height in the fossil record. But the work does highlight how even seemingly simple physical features have complex evolutionary histories.
November 28, 2012
One of the great origin stories in the history of mammals is the rise of primates. It’s a story that scientists are still trying to write.
In the early 20th century, anatomists believed primates—united by big brains, grasping hands and feet, and excellent vision, among other features—evolved in response to living in trees. In the 1970s, however, biological anthropologist Matt Cartmill realized an arboreal lifestyle alone wasn’t enough to explain primates’ unique set of characteristics. Plenty of mammals, like chipmunks, live in trees but don’t have nimble hands or closely spaced, forward-facing eyes that allow for good depth perception. Instead, Cartmill suggested these features evolved because early primates were insect predators. He noted that many modern predators, such as cats and owls, have forward-facing eyes because they rely on good vision to grab prey. In the case of early primates, Cartmill said, they hunted tree-dwelling insects.
Not long after Cartmill presented his explanation of primates’ roots, other researchers came up with an alternative idea: Primates evolved in step with the spread of flowering plants. Rather than relying on good vision and dexterity to nab bugs, early primates used these traits to carefully walk out to the ends of delicate tree branches to gather fruits and flowers, as well as the insects that pollinated flowering plants.
Physical anthropologists Robert Sussman and D. Tab Rasmussen of Washington University and botanist Peter Raven of the Missouri Botanical Garden review the latest evidence in support of this hypothesis in an article published online in the American Journal of Primatology.
The team suggests that the earliest primates and their extinct close relatives, a group called plesiadapiforms, weren’t strictly insect eaters and therefore the insect predation hypothesis doesn’t hold up. They point out that the molars of plesiadapiforms are rounder than the teeth of earlier mammals, which were sharp for puncturing bugs. The flatter teeth indicate plesiadapiforms were probably grinding fruits, nuts and other plant parts.
The switch to a plant diet coincides with the rise of rise of flowering plants. The earliest flowering plants show up in the fossil record roughly 130 million years ago and became the dominant type of forest plant by about 90 million years ago. Around 56 million years ago, global temperatures spiked and tropical forests spread around the world. About this time, many species of birds and bats emerged. Primates also diversified during this period. Sussman and his colleagues argue that while birds and bats could fly to the ends of branches to consume meals of fruit and nectar, primates took a different route, evolving adaptations that enabled them to be better climbers.
The skeleton of a 56-million-year-old plesiadapiform found in Wyoming provides further evidence of this scenario, the researchers say. Much of the early primate and plesiadapiform fossil record consists of teeth, but in 2002, scientists reported the discovery of the skull, hands and feet of Carpolestes simpsoni. The bones reveal that the species was a good grasper, with an opposable big toe and nails instead of claws. And the teeth indicate the creature ate fruit. But unlike living primates, C. simpsoni did not have forward-facing eyes, suggesting it didn’t have good depth perception. This is an important finding, Sussman and colleagues say. If primates evolved their characteristic features because they were visual predators, then you’d expect good vision to evolve in concert with good grasping. Instead, the C. simpsoni fossils suggest enhanced vision came later. Forward-facing eyes may have later evolved because it helped primates see through the cluttered, leafy environment of the forest canopy.
The team’s arguments rest heavily on evidence from plesiadapiforms. In the past, anthropologists have debated plesiadapiforms close connection to primates. However, Sussman and colleagues think the fossil evidence suggests the two groups shared a common ancestor, and thus the evolutionary trends seen in plesiadapiforms serve as a good guide for what happened in primates.
November 14, 2012
The nearly 2-million-year-old Paranthropus boisei was the cow of the hominid family. Unlike other human cousins, the species was a fan of dining on grasses. But it turns out it wasn’t the only, or even the first, hominid grazer. Australopithecus bahrelghazali was munching on grasses and sedges at least 1.5 million years before the origin of P. boisei, a new study in the Proceedings of the National Academy of Sciences suggests. The findings may mean early hominids were capable of consuming a wide variety of foods and colonizing new environments.
But before we discuss how scientists figured out A. bahrelghazali‘s diet, and why that matters, we need to address a far more pressing question: Who the heck was A. bahrelghazali?
In 1993, researchers in Chad unearthed a 3.5-million-year-old hominid lower jaw fragment and a few attached teeth. Based on the fossils’ age, many paleoanthropologists think the bones belonged to Australopithecus afarensis. But the specimen was found more than 1,500 miles farther west than any other A. afarensis bones, and subtle differences in the size and shape of the fossils led the discoverers to conclude they had found a new species. They named it A. bahrelghazali after the Bahr el Ghazal valley in Chad where the bones were recovered. Since then, researchers haven’t found any other A. bahrelghazali fossils and its species’ status remains controversial.
With just a jaw and teeth, there’s not too much scientists can say about what A. bahrelghazali looked like or how it lived its life. But, fortunately, diet is something that can be gleamed from these fossils. Analyzing the teeth’s chemistry is one way to assess what the species ate. This is possible because the carbon found in plants comes in two versions, or isotopes, called C3 and C4. Trees and other forest plants are rich in C3; grasses, sedges and other grassland plants have an abundance of C4. When an animal eats these plants—or eats other animals that eat these plants—the different carbon isotopes get incorporated into the individual’s teeth, serving as a record of what it once ate. Previous work on P. boisei has shown that C4 plants made up as much as 77 percent of that hominid’s diet.
In the new study, Julia Lee-Thorp of Oxford University and colleagues come to a similar conclusion for A. bahrelghazali, that the species mainly ate C4 plants, probably grasses and sedges. And like modern baboons that live on savannas, the hominid probably ate different parts of these plants, including underground tubers and bulbs. This diet is not surprising given the type of habitat A. bahrelghazali lived in. Based on the other types of animals found near the hominid, the researchers say A. bahrelghazali made its home in an open grassland, with few trees, near a lake. So forest foods weren’t really a dining option.
The results mean that by 3.5 million years ago hominids were probably already “broad generalists” capable of eating a variety of foods depending on what was locally available, the researchers say. (The younger Australopithecus sediba,which lived roughly 2 million years ago, demonstrates some of the stranger foods that hominids could eat: The South African species liked to eat wood—a dietary preference not seen in any other hominid.) Being a food generalist may have allowed A. bahrelghazali to explore new environments and leave behind the forests that earlier hominids, such as Ardipithecus ramidus, and their ancestors resided in.
September 19, 2012
I was intrigued when I saw this headline over at NPR’s 13.7 blog earlier this week: “A Neanderthal-Themed Park for Gibraltar?“ As it turns out, no one’s planning a human evolution Disney World along Gibraltar’s cliffs. Instead, government officials are hoping one of the area’s caves will become a Unesco World Heritage site. Gibraltar certainly deserves that distinction. The southwestern tip of Europe’s Iberian Peninsula, Gibraltar was home to the last-surviving Neanderthals. And then tens of thousands of years later, it became the site of one of the first Neanderthal fossil discoveries.
That discovery occurred at Forbes’ Quarry in 1848. During mining operations, an officer in the British Royal Navy, Captain Edmund Flint, uncovered an adult female skull (called Gibraltar 1). At the time, Neanderthals were not yet known to science, and the skull was given to the Gibraltar Scientific Society. Although Neanderthals were recognized by the 1860s, it wasn’t until the the first decade of the 20th century that anatomists realized Gibraltar 1 was indeed a Neanderthal. Additional Neanderthal discoveries came in the 1910s and 1920s at the Devil’s Tower rock shelter, which appeared to be a Neanderthal occupation site. In 1926, archaeologist Dorothy Garrod unearthed the skull of a Neanderthal child near flaked stone tools from the Mousterian industry. In all, archaeologists have found eight Neanderthal sites at Gibraltar.
Today, excavations continue at Gorham’s Cave and Vanguard Cave, where scientists have learned about the life and times of the most recent populations of Neanderthals. In 2006, researchers radiocarbon dated charcoal to estimate that the youngest Neanderthal populations lived at Gibraltar as recently as 24,000 to 28,000 years before the present. Clive Finlayson, director of the Gibraltar Museum’s Heritage Division, has suggested that Neanderthals persisted so late at Gibraltar because the region stayed a warm Mediterranean refuge while glacial conditions set in across more northern Europe. Ancient pollen data and animal remains recovered from Gibraltar indicate Neanderthals had access to a variety of habitats—woodlands, savannah, salt marshes and scrub land—that provided a wealth of food options. In addition to hunting deer, rabbits and birds, these Neanderthals enjoyed eating monk seals, fish, mussels and even dolphins on a seasonal basis.
As with most things in paleoanthropology, the Neanderthal history at Gibraltar is not settled. Some anthropologists have questioned the validity of the very young radiocarbon dates. Why the Neanderthals eventually died out is also a matter of debate. Further climate change in Europe, competition with modern humans or some mix of both are all possible explanations.
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.