Darwin had a straightforward explanation: People, moles, horses, porpoises and bats all shared a common ancestor that grew limbs with digits. Its descendants evolved different kinds of limbs adapted for different tasks. But they never lost the anatomical similarities that revealed their kinship.
The fossil record shows that we share a common aquatic ancestor with ray-finned fish that lived some 430 million years ago. Four-limbed creatures with spines — known as tetrapods — had evolved by 360 million years ago and went on to colonize dry land.++
His discoveries include a 370 million-year-old fish called Tiktaalik, which had limb-like fins. It developed endochondral bones corresponding to those in our arms, beginning at the shoulder with the humerus, then the radius, ulna and wrist bones. But it lacked fingers, and still had a short fringe of fin rays.
When he is not digging for fossils, Dr. Shubin runs a lab at the University of Chicago, where he and his colleagues compare how tetrapods — mice, for example — and fish develop as embryos.
Their embryos start out looking very similar, consisting of heads and tails and not much in between. Two pairs of buds then develop on their flanks. In fish, the buds grow into fins. In tetrapods, they become limbs.
The new discovery could help make sense of the intermediate fish with limb-like fins that Dr. Shubin and his colleagues have unearthed. These animals still used the molecular addresses their ancestors used. But when their cells reached their addresses, some of them became endochondral bone instead of fin rays. It may have been a simple matter to shift from one kind of tissue to another.
“This is a dial that can be tuned,” Dr. Shubin said.
http://www.nytimes.com/2016/08/18/s...a-deep-evolutionary-link.html?ref=todayspaper
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