Ancient fish sheds light on vertebrate eye evolution

Using CT scanning and X-ray microchromatography, palaeontologist Carole Burrow and her colleagues have produced three-dimensional reconstructions of the eye of the placoderm, a jawed fish which lived about 410 million years ago during the early Devonian period.

The fossilized placoderm specimens, found in limestone deposits in new South Wales, Australia, had intact, but very fragile, eyes. By scanning 7 micrometre-thick slices of the eyeball, detailed images of the placoderm eye were obtained.

From the images, Burrow could distinguish scars left where muscles had been attached to the eyes, as well as holes through which nerves and blood vessels passed.

Whereas all known jawed vertebrates have six muscles controlling eye movement, it was found that placoderms had seven. The seventh muscle carried out the same function as the others, but was positioned uniquely. It was controlled by one of three cranial nerves, which would, during placoderm development, have grown over the other two so that it was closer to the front of the head.

Gavin Young, who worked with Burrow on the study, suspects that the extra muscle may have something to do with the fact that the placoderm eye was attached to the skull by a stalk.

Although placoderms are thought not to have any extant descendants, the findings, which are published in the journal Micron, should help biologists to better understand how the vertebrate eye evolved.

What will the neanderthal genome teach us about human brain evolution?

Palaeoanthropologists at the Max Planck Institute, in collaboration with scientists at 454 Life Sciences Corp., of Branford, Connecticut, have begun a two-year project to sequence the neanderthal genome. The start of the Neanderthal Genome Project coincides with the 150th anniversary of the discovery of the specimen-type Homo neanderthalensis fossil in the Neander valley near Dusseldorf, Germany.

The ancestor of the chimpanzee, our closest living relative, split from ours about 7 million years ago. According to archaeological, palaeontological and molecular genetic data, modern humans and neanderthals both evolved from Homo erectus, with the two lineages diverging about 300,000 years ago. These data also support the hypothesis that modern humans arose relatively recently in Africa, and suggest that neanderthals were unlikely to have interbred with, or contributed any genes to, modern humans.

Neanderthals lived in Europe and Western Asia between approximately 300,000 to 28,000 years ago. They are believed to have been relatively sophisticated, but although there is evidence that neanderthals formed groups and buried their dead, they lacked many of the cognitive functions, such as higher reasoning, that we have. H. neanderthalensis was therefore either out-competed or driven to extinction by modern humans, whose emergence coincided with the disappearance of neanderthals.

We share 99% of our DNA with chimps. According to 454 Life Science’s Neanderthal Press Kit, “it is estimated that the Neanderthal shares 96% of the 1% difference with Homo sapiens. The neanderthal shares the remaining 4% of the difference with the chimpanzee.”

In other words, neanderthals were closer to us than they were to chimps, and, of all the hominid species that have existed, are the closest to humans. Sequencing and mapping of the neanderthal genome will therefore hopefully shed light on the relationship between that species and our own, and on evolution of the human brain.