Fishy Ancestors of Humans Surprisingly Diverse

Ker Than
for National Geographic News

June 25, 2008

The fishlike ancestors of humans and other land animals were a surprisingly diverse bunch, according to a new fossil reconstruction of the transitory species Ventastega curonica.

The aquatic creature, which lived during the late Devonian period about 365 million years ago, represented an evolutionary midpoint between Tiktaalik, one of the earliest fish to clamber onto land, and primitive four-legged land animals, or tetrapods.

"Ventastega gives clues to what the very earliest tetrapods looked like," said study leader Per Ahlberg of Uppsala University in Sweden.

Ventastega was first described from a few bone fragments unearthed in Latvia in 1994, but it took additional years of excavation and the discovery of remains from many more individuals before scientists had a good idea of what the creature looked like.

The latest portrait to emerge, from an especially well-preserved find, reveals an animal with a part-fish, part-tetrapod skull and a full-fledged tetrapod body. It would have spent the majority of its time on water and been clumsy on land.

While Ventastega in many ways fulfills scientists' expectations of what an early water-land transition animal should look like, it also shows substantial morphological differences from other similar species living around the same time.

These differences could help explain the seemingly swift rate at which later land animals diverged into separate species, scientists say.

"It's kind of remarkable that when we start to see true limbed animals, they've already diversified and are filling various special niches within this aquatic ecosystem," said Ted Daeschler, a paleontologist at the Academy of Natural Sciences in Philadelphia not affiliated with the new study.

The work is detailed in the June 26 issue of the journal Nature.

Third Example

Prior to Ventastega's discovery, scientists had characterized the earliest tetrapods mostly on fossils of two primitive examples: Acanthostega and Ichthyostega.

Both land creatures lived during the late Devonian. But they looked nothing like each other.

"Were the very first tetrapods like Ichthyostega or like Acanthostega or like neither?" study author Ahlberg asked.

Ventastega provides a valuable third example. Its features are similar to those of Tiktaalik and Acanthostega but do not resemble Ichthyostega's at all.

"This suggests that Ichthyostega is a specialized offshoot of early tetrapod evolution, whereas Ventastega and Acanthostega are closer to the main line of transition from water to land," Alhberg said.

Awkward Creature

Ventastega was likely not entirely comfortable in water or on land. But the animal already bore some of the adaptations that would allow its descendants to one day abandon the sea completely.

Its shoulder girdle, where the forelimbs attached to the body, had acquired the characteristic tetrapod shape, and the creature's pelvis was attached to the backbone, suggesting Ventastega's paired hind fins had already turned into limbs.

In addition, the shape of its head and the pattern of its teeth are intermediate between those of Tiktaalik and Acanthostega, hinting at a gradual change in feeding behavior.

Ventastega is one of about a dozen known fossil species representing the transition from fish to tetrapods, comparable to the number known for the evolution of land mammals into whales, scientists point out. (Related: "Whales Evolved From Tiny Deerlike Mammals, Study Says" [December 19, 2007].)

So researchers have a rough idea of the major evolutionary changes that took place but still have their work cut out for them when it comes to filling in the gaps.

Daeschler, of the Academy of Natural Sciences, compared studying the fish-to-tetrapod transition to building a house.

"We've got the frame built. We know what the rooms are shaped like," he said. "But we haven't put in the electricity, installed the lamps, or put Sheetrock on the walls."