For the 200th anniversary of Charles Darwin's birth (February 12, 2009), National Geographic News asked leading scientists for their picks of the most important fossils that show evolution in action—seven of which are presented here, starting with this "fishapod."

Tiktaalik: The "Fishapod"

Discovered in Arctic Canada in 2004, 375 million-year-old Tiktaalik had not only gills and scales but traits of a tetrapod (four-legged land animal), including limblike fins, ribs, a flexible neck, and a croc-shaped head.

Why it matters: Tiktaalik is seen as evidence of the period when our aquatic ancestors began moving ashore—along with other fins-to-limbs fossils, such as Acanthostega (Acanthostega picture), the most primitive known tetrapod.

Early Darwin supporters speculated that such fishes had given rise to amphibians. "Acanthostega and Tiktaalik have taken this to a new level," said geologist Donald Prothero, of Occidental College in Los Angeles.

The discoveries of these and other "missing link" species have helped dispel what Darwin called perhaps "the most obvious and gravest objection which can be urged against my theory" of evolution--the former lack of transitional fossil species.