The discovery of a 160 million-year-old fossil, Limusaurus inextricabilis, has offered scientists a glimpse of how the three-fingered hand evolved in birds. This particular issue has gained the interest of not only paleontologists, but also evolutionary and developmental biologists.
The fossil, which was discovered in a mine in northwestern China, is a primitive ceratosaur, a part of the theropod group that also included the Tyrannosaurus rex. As Professor Clark described, “It’s a really weird animal – it’s got no teeth, had a beak and a very long neck, and very wimpy forelimbs.” The fossil possessed a truncated first finger (thumb), middle three fingers, and no fifth finger. Whereas scientists previously thought that the first three digits persisted among three-fingered animals, this recent finding indicates that it was the middle digits that persisted. (This coincides with the findings of developmental biologists, who have shown bird embryos display growth in all five digits until the first and fifth are reabsorbed.)
This discovery also provides a glimpse into “identity shift,” or the shifting of gene expression from one limb to another. In this case, expression is shifted from the first to the second finger. Thus, one can see how paleontologists mistakenly believed that the second finger was a vestige of the first finger.
According to Jack Conrad, a paleontologist with the American Museum of Natural History, “This is amazing – it’s the first time we’ve seen this thing actually starting to disappear. There’s been this fundamental rift – there was no way to make peace between the good data we were seeing from the developmental biologists and the palaeontological evidence that showed with every fossil we found we were seeing [fingers] one, two and three.”
It appears that the developmental biologists were right on this one. However, if science is to truly blossom and grow as a field, specialists from all fields must be brought to bear on the major scientific questions of the day. In the nineteenth century, we needed geneticists to inform and restructure evolutionary theory. Now, we will need to look to the burgeoning fields of developmental biology, neurobiology, and epigenetics.
For BBC article, go here.