University of Ottawa researchers Jing Zhang and Marie-Andrée Akimenko have discovered that if the proteins actinoden 1 & 2 are suppressed in fish, fin development gives way to something like leg development. These two proteins are found in fish, but not in reptiles, mammals, or other tetrapods. (discussion, original article). The date of this original transition is immensely far into the past, and new research continues to push it back into the temporal gulf.
That such a small alteration is all that governs such a huge change should not be surprising to those familiar with the history of Innsmouth and similar communities.
"everything alive come aout o' the water onct an' only needs a little change to go back agin."
- Zadok Allen, Innsmouth Massachusetts, July 15, 1927
One wonders what the researchers would find if they were able to examine biological samples collected by the federal investigation of Innsmouth.
UPDATE: Meet Tiktallik, the 375 million year old "fishapod" discovered six years ago. It has all the basic structures that will form the human body. Perhaps we should instead call it Dagon?