by Erin Wayman Thursday, January 5, 2012
Last October, scientists formally introduced the world to Ardi the Ardipithecus, the well-preserved skeleton of a 4.4-million-year-old hominin found in Ethiopia. Eight months later, scientists have had time to digest the data from all 11 papers that were published in Science last fall regarding Ardi’s biology and ecology, and there is some dissent.
In a paper published in Science today, Thure Cerling, a professor at the University of Utah in Salt Lake City who studies stable isotope ecology and paleoecology, and seven colleagues find fault with the original interpretation of Ardi’s paleoenvironment. Ardi’s discoverers say she lived in a woodland forest, with monkeys, porcupines and bats for neighbors. Cerling et al. say the paleoecological evidence — which includes stable carbon isotopes recovered from ancient soil, carbon isotopes retained in fossil teeth, and the type and abundance of animals found in association with Ardi — actually suggests that Ardi lived in a “bush-savanna,” a grassland with 25 percent or less forest cover.
Tim White, the University of California at Berkeley paleoanthropologist who led the analysis of Ardi, and colleagues have responded to this criticism by agreeing that Ethiopia was home to a range of habitats 4.4 million years ago, including wooded grasslands. But, they say, the bulk of evidence shows that Ardipithecus preferred to reside in the forest.
So, why does this quibble matter? Well, last fall, when White and colleagues claimed Ardi lived in a forest, they said the finding would once and for all put an end to the savanna hypothesis — one of the main explanations for why our ancestors evolved upright walking, or bipedalism.
The crux of the savanna hypothesis is that bipedalism became an advantageous form of walking when Africa’s forests shrank and were replaced by open grasslands (some of the earliest incarnations of the idea suggested standing upright would help our ancestors see over the grass to spy predators). This hypothesis can be traced back to Raymond Dart, an anatomist at the University of Witwatersrand in South Africa, who in 1924 discovered the first Australopithecus specimen. The specimen, dubbed the Taung Child, appeared to have lived in a savanna.
Ardi is not the first challenge to this hypothesis, however. Starting in at least the 1970s, accumulating fossil and paleoecological evidence showed that early hominins lived in at least partially wooded environments. In fact, Lucy’s species, Australopithecus afarensis, retained some anatomical features related to climbing, which some researchers say is evidence that the species spent time in trees.
Such evidence led to modifications of the savanna hypothesis, with some anthropologists (including Henry McHenry and my graduate school advisor, Peter Rodman, both now retired from the University of California at Davis) suggesting upright walking was an efficient way for early hominins to travel between clumps of forest when grasslands were first expanding.
In 1981, C. Owen Lovejoy, an anthropologist at Kent State University in Ohio, came up with an explanation for the origin of bipedalism that didn’t involve the savanna; instead, he said, walking upright co-evolved with monogamy.
This is a summary of the scenario envisioned by Lovejoy: Early hominins were omnivorous apes who lived in forests. As the environment became more seasonal and climatic changes led to more variable environmental conditions, it became harder for our ancestors to find food; they had to spend more time looking for grub and they had to travel farther to get it.
This was especially a drag for females, who had to bring their offspring with them wherever they went. That was also dangerous, as the greater search for food could expose females and their infants to more hungry predators. But males and females found a mutual solution to this problem: food-for-sex provisioning. Males would go off and gather food for a female and her offspring. In turn, females would mate exclusively with that male and take care of his babies. In essence, Lovejoy was suggesting the classic 1950s nuclear family was born in a dark forest in Africa.
Where does bipedalism fit in? It’s a lot easy to gather food and carry it back to your “family” if your arms are free, Lovejoy reasoned, so walking on two legs was an essential adaptation for monogamy to work. Along with this scenario, males no longer needed to fight with each other over access to females; therefore, males' sharp, dagger-like canine teeth — ubiquitous in the ape world — became smaller and smaller over time.
Like many hypotheses regarding bipedalism, and human evolution in general, the food-for-sex explanation was not universally accepted back in the early 1980s. But when Ardipithecus was found in a forest setting — and was an omnivore with small canine teeth — Lovejoy must have felt vindicated, for he wrote an article in Science (one of the 11 papers) that basically restated his argument from 30 years earlier.
And like 30 years ago, it seems that not everyone is backing the idea. In their Science paper, Cerling and colleagues don’t endorse the savanna hypothesis outright, but they note that “the connection between bipedalism and … grass expansion … remains a viable idea.” It looks like the savanna hypothesis is not yet dead after all.
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