Researchers used a clever trio of tricks to figure out what 2 million-year-old human ancestors ate by analyzing the stuff on their teeth, and they found something unexpected: They ate more like chimpanzees than like humans. Their analysis could point to a reason why our species triumphed while some of our long-lost cousins failed.

"Our results suggest that there was more variation among hominins around 2 million years ago, in terms of what they ate and where they lived," Amanda Henry of Germany's Max Planck Institute for Evolutionary Anthropology explained in an email, using the technical term for humans and their close fossil relatives.

The various species spread out into different environments, ranging from forests to grasslands, but each species had a preferred environment, in part due to diet specialization. "Then, with the evolution of Homo erectus around 1.8 million years ago, we see a big shift," Henry said. "Homo erectus thrived in a whole variety of environments, and was even able to migrate out of Africa."

Homo erectus eventually gave rise to Homo sapiens — that is, us. Meanwhile, the hominins that were tied down to Africa's forests — perhaps including Australopithecus sediba, the South African species that Henry and her colleagues studied — faded away in the competition with other apes. The latest research, published online today by the journal Nature, doesn't address that big evolutionary issue directly. But it does provide plenty of food for thought.

Decoding the diet
Australopithecus sediba was discovered in 2008, and has come to be seen as a transitional species between the relatively small, ape-like pre-humans known as australopiths and our closer ancient relatives in the genus Homo. Only a few specimens of the species have been recovered, but they appear to reflect chimp-like as well as human-like characteristics.

So which way did Au. sediba swing when it came to its diet? The researchers focused on the teeth of two specimens, a juvenile male and an adult female.

First, the researchers zapped the tooth enamel with a laser and analyzed the vapor that was given off. The composition of the enamel preserves a record of what the individual ate while it was growing up. In this case, Johns Hopkins University geochemist Benjamin Passey used a mass spectrometer to look at the balance of carbon-12 and carbon-13 isotopes in the vapor.

Lee Berger

The teeth of a juvenile male Australopithecus sediba had a dark layer of calculus, which is clearly visible in this close-up picture.

If the level of carbon-12 is relatively high, that suggests a diet heavy in forest products such as leaves, fruits and the critters that eat those things. Those are known as C3 foods. But if carbon-13 is high, that points to foods from the African savanna, such as seeds, roots and grasses. These are C4 foods.

"It seems like a hallmark of human evolution to be able to use savanna resources," Passey told me. "Today, most of our energy comes from grass in one way or another, either from grain or from animals that eat grain and grass."

Tests on sediments and various animal bones suggested that the area where the specimens were found was a savanna 2 million years ago. However, when Passey analyzed the samples from Au. sediba, he found that the carbon was almost pure C3, suggesting a diet heavy in forest products.

Two other lines of evidence confirmed that preference: Deposits of plaque on the teeth contained bits of silica, known as phytoliths, that were linked to forest vegetation. Also, the pits on the teeth were characteristic of creatures that favored a forest diet. All the evidence pointed to the conclusion that these particular pre-humans went to the forests for virtually all of their food, even though their remains were left behind in the savanna.

"This astonished us," Passey said.

The meaning of diet
Upon reflection, it shouldn't be that surprising that Australopithecus sediba passed up what the African savanna had to offer, said the University of Colorado's Matt Sponheimer, another co-author of the Nature paper. "We know living apes don't seem to like such foods much — or at least they are never a large portion of their diets," he told me in an email. "It is likely that this is at least partly due to their masticatory (and probably digestive) anatomy being suboptimal for such diets."

Modern-day savanna chimpanzees engage in similar behavior, the researchers noted.

The findings are consistent with the view that "early hominins were quite flexible with respect to diet, with different populations preferring different parts of the available plant resource base, depending on when they lived, and where," said Bill Kimbel, who serves as director of Arizona State University's Institute of Human Origins but was not involved in the Nature study. "The plant phytoliths are a nice addition to the repertoire of perspectives on the issue."

In an email exchange, Kimbel cautioned against reading too much into Au. sediba's preference, as reflected by the results from two individuals. He took issue with my suggestion that the results put the species in the "chimp camp" rather than the "human camp," and noted that other hominin species had different preferences for C3 vs. C4 foods.

"I don't think sediba should be seen as 'remarkable' in this context," he told me. "And sediba no more belongs in the 'chimp camp' than it does in the 'giraffe camp' (with which it also shares dominant C3 values). This is not a useful analogy."

Sponheimer declined to say whether he thought Australopithecus sediba died out because it didn't shift its diet to the foods of the savanna, "but one could make such an argument if indeed this was an organism tied to very specific microhabitats by a limited dietary repertoire." A different argument could be made as well: Perhaps the descendants of Au. sediba eventually adapted to a diet that took in C4 as well as C3 foods, and thus contributed to the rise of early Homo.

Passey, for one, would love to study more specimens. "It would be nice to analyze those and see if all members of that species have that same forest behavior," he told me. However those future experiments turn out, the findings reported today show how novel techniques — ranging from precision laser blasting to tooth-crud analysis — can shed light on the origin of our species.   

More about human origins:

• Was there a fork in our family tree?
• Humans had sex with now-extinct relatives
• Fossils shake up humanity's family tree
• Savannas may have nurtured human ancestors

In addition to Henry, Sponheimer and Passey, the authors of "The Diet of Australopithecus Sediba" include Peter S. Ungar, Lloyd Rossouw, Marion Bamford, Paul Sandberg, Darryl J. de Ruiter and Lee Berger. The paper, published online today, will appear in a future print edition of Nature.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Darryl de Ruiter

This specimen, known as SK 48, is one of the best examples of Paranthropus robustus from South Africa's Swartkrans Cave.

An analysis of fossil teeth from South Africa suggests that the males in pre-human societies stayed near the caves where they grew up, while the females migrated when it was time to mate.

The researchers behind the analysis say their findings, published in this week's issue of the journal Nature, could shed light on the migratory behaviors that eventually gave rise to human societies.

"This appears to be most similar to a chimpanzee social structure," said lead author Sandi Copeland, an anthropologist at the University of Colorado and Germany's Max Planck Institute for Evolutionary Anthropology. It's also consistent with the way many humans handle the process of moving out and settling down, she told journalists. "It's more often than not in modern societies that the woman is the one that leaves," she noted.

But it's not the norm in the animal world. "In most primates and most mammals, it's usually the males who leave their home community," she said. Thus, the analysis could hint at one of the factors that made us human.

Darryl de Ruiter

The skull of "Mrs. Ples" is the most famous example of Australopithecus africanus from South Africa's Sterkfontein Cave.

The research is the result of a "continuing effort that we have been making over the last five to 10 years," focusing on an area of South Africa that's rich with caves and pre-human fossils, said Oxford University archaeologist Julia Lee-Thorp, another one of the study's co-authors. She and her colleagues have been measuring the radioisotope distribution around the Sterkfontein and Swartkans cave sites, to come up with the chemical fingerprints for samples in that area.

The point behind this exercise is to chart out the migration patterns for species that can no longer tell their own tales. By sampling the isotopes of strontium in tooth enamel, the team could determine which teeth belonged to animals that grew up in the cave region and absorbed strontium in the characteristic isotopic proportions from the foods they ate. That chemical signature remained intact in the tooth enamel throughout adulthood, even if the animals later roamed to a different area.

Sandi Copeland

Faint marks, left behind by a laser ablation procedure, can be seen along the right side of this Australopithecus africanus molar.

The researchers carefully blasted 19 pre-human teeth with pinpoint laser beams, and then analyzed the results. Eight of the teeth came from the Sterkfontein Cave and were traced to a species known as Australopithecus africanus, dating back about 2.2 million years. The other 11 teeth came from the Swartkans Cave, and are attributed to Parathropus robustus, a species that lived about 1.8 million years ago. Australopithecus is thought to be a closer ancestor of modern humans than Paranthropus.

The team expected to see a difference in the local vs. non-local distribution of the two species. They didn't.

"As the numbers rolled off the mass spectrometer after each laser ablation, we were at first disappointed," Lee-Thorp said in an Oxford news release. "But we soon realized that we had found another prize — a difference between the males and females. It was totally unexpected."

The researchers assumed that nine significantly larger teeth in their sample came from males, while the 10 significantly smaller teeth came from females. They chose these teeth for comparison precisely because of the size difference.

Based on the strontium analysis, more than half of the female-sized teeth came from outside the cave region, while 90 percent of the male-sized teeth were traced to the caves. That finding led the researchers to conclude that the males were stay-at-home types, while the females were more likely to roam.

Such a pattern is similar to that seen in chimpanzee societies, where males in a particular locale tend to stick together to defend their turf from interlopers. In order to guard against inbreeding, the younger females are likelier than the males to migrate for mating. Copeland said the situation is different for gorillas. In those societies, the dominant male gorilla rules over a harem that tends to stay put, while younger males usually have to go someplace else to find their own mates.

"This study gets us closer to understanding the social structure of ancient hominds, since we now have a better idea about the dispersal patterns," Copeland said in a University of Colorado news release.

The team's conclusions are based on less than two dozen teeth, divided up into species and sex categories. That raised questions about the small sample size. Texas A&M anthropologist Darryl de Ruiter, a study co-author, acknowledged that the team was "very constrained by the amount of material that we have available for destructive analysis." But he said that adding a few more teeth to the sample size may not have helped, because they had already selected the largest and the smallest teeth available to them.

"Anything else we would have added would have been in the gray area," he told journalists. "It wouldn't have added any value to the discrimination between males and females."

More about human origins:

• Lucy's 'great-grandfather' found
• How big babies shaped society
• Can fingers point to sex habits?
• Interactive: Before and after humans

In addition to Copeland, Lee-Thorp and de Ruiter, co-authors of "Strontium Isotope Evidence for Landscape Use by Early Hominins" include Matt Sponheimer, Daryl Codron, Petrus J. le Roux, Vaughan Grimes and Michael P. Richards.

You can connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. Also, give a look to "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.