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Bone collectors dig into our past

by Rowan Hooper

Two papers published today shed light on our early evolution, though “early” is a relative term. The first describes what could’ve been the first species of mammal, a tiny beast that quivered in the shadows of the dinosaurs 195 million years ago. The second reports on a shift in eating habits of early modern humans compared to Neanderthals about 25,000 years ago.

A team of U.S. and Chinese researchers has identified a new species of primitive mammal, uncovered from Early Jurassic rocks in the Lower Lufeng formation, in Yunnan, China, dating to 195 million years ago. The new fossil species, Hadrocodium, has distinctly mammalian features, predating the previous oldest-known mammal-like specimen by 45 million years.

Zhe-Xi Luo, of the Carnegie Museum of Natural History, and colleagues describe the find — a 12-mm-long skull — in today’s issue of Science. Extrapolating from the skull size, the researchers estimate that Hadrocodium weighed only 2 grams — the size of a large paper clip. The smallest living ground-dwelling insectivorous placental mammal weighs 2.5 grams, the smallest bat, 2 grams.

There are several key differences in the evolutionary transition from mammal-like reptile to mammal, including changes to the jaw and brain. Reptiles have a lower jaw made from several bones, whereas in mammals three of the jaw bones form the middle ear bones and the lower jaw is a single bone. Hadrocodium conforms to the mammalian type. Hadrocodium also had a large brain vault, and a CT scan of the fossil shows that the olfactory lobes were particularly well-developed.

These are key mammalian innovations, and their presence in this fossil suggests that they evolved step by step, long before the differentiation of living mammal forms. Luo and colleagues choose the name Hadrocodium (hadro meaning “full,” and codium meaning “head”) to reflect one of its mammalian features. They suggest that the expanded brain contributed to the separation of the ear bones from the jaw.

So here we have a skull that tells us that this animal had a large cranial cavity, sensitive hearing and a powerful jaw for chewing. A missing link?

“Hadrocodium could be our distant cousin, an early animal that lived alongside the ancestor of living mammals,” said Luo. “Or it could be our great-great-granduncle, closely related to living mammals but not in our direct lineage. Or Hadrocodium could be the direct ancestor of living mammals.”

From the fossil evidence we can’t say for sure. “But we are satisfied to know that Hadrocodium is the sister taxon to all living mammals,” Luo added.

Millions of years after Hadrocodium and its relatives emerged from the shadows of the giant reptiles, our later ancestors gradually replaced another, less formidable but much more closely related foe: Neanderthals.

Michael Richards, of the University of Bradford, U.K., and colleagues analyzed the bones of nine early modern human skeletons found in Europe and western Asia, and compared the results with what they had found last year about Neanderthals from that geographic area.

Neanderthals, as we know, were hard. They were the burly, heavy-browed meatheads of prehistory. Using bone-chemistry analysis, Richards and his team showed last year that Neanderthals had a diet appropriate to their image, and one similar to the other top-level predators of the day — wolves and lions. They ate, almost exclusively, meat, perhaps including mammoths. In contrast, the team now shows that early modern humans had a far broader dietary spectrum than Neanderthals.

Richards and his colleagues used an isotope-analysis technique that measures the amounts of carbon and nitrogen isotopes in the bone collagen of skeletons. The technique allows researchers to determine the average dietary protein intake of the individual over the 10 years before death. It means we don’t have to rely just on tools and artifacts found at archaeological digs to build a picture of our ancestors’ lives.

The team analyzed skeletons dating from the mid-Upper Paleolithic, 20,000-28,000 years ago, from locations in Britain, the Czech Republic and Russia. The isotope data revealed that these early moderns had a diet largely based on fish, mollusks and waterfowl. Combined with other archaeological data, the results have implications beyond merely an appreciation of what was on the menu for our early ancestors.

Neanderthals ate “highly ranked game”: animals that have a high energy or protein yield relative to the effort required to catch and process them. In other words, they ate big animals that gave big returns. The diet of early moderns was one of “lower-ranked game”: small, quick-moving animals like fish that required technology (e.g., nets) to capture, but also posed less of a threat, thus making them more economical to hunt and eat.

Therefore, not only were early moderns more technologically advanced than Neanderthals, but they had the advantage of a more or less constant food supply. Populations of large game vary with the weather and are often rare. “Neanderthals would have been more susceptible to seasonal and annual resource fluctuations,” say the authors.

They might’ve been tough, but their red-meat diet left them at the mercy of natural pressures, and they were gradually replaced by the tool-using, fish-eating ape, Homo sapiens.