More Early Tetrapod Fossils Help Uncover the Origin of Life on Land

June 25, 2008

Another interesting specimen uncovered in the sprawling family tree of the early tetrapods (the four-limbed fishy conquers of land): Ventastega curonica.

It’s worth noting, yet again, how fossil evidence generally works when it comes to evolution. Discerning the exact ancestry of most specimens is generally implausible… but that’s not really what scientists require in the first place. What they want and need is to flesh out the overall family tree:

Scientists don’t think four-legged creatures are directly evolved from Ventastega. It’s more likely that in the family tree of tetrapods, Ventastega is an offshoot branch that died off, not leading to the animals we now know, Ahlberg said.

“At the time, there were a lot of creatures around of varying degrees of advancement,” Ahlberg said. They all seem to have similar characteristics, so Ventastega’s find is helpful for evolutionary biologists.

Of course, some people, such as the science journalists that penned the piece, seem a little rusty on the concept:

Ventastega is the most primitive of these transition animals, but there are older ones that are oddly more advanced, said Neil Shubin, professor of biology and anatomy at the University of Chicago. He was not part of the discovery team but helped find Tiktaalik, the fish that was one step earlier in evolution.

“It’s sort of out of sequence in timing,” Shubin said of Ventastega.

Shubin’s likely point here, rather poorly represented in the article, is not that we’d necessarily expect to see Ventastega specifically “in sequence,” but simply that it isn’t on the direct line from which all land animals are descended.

To many lay readers, however, the “primitive” and “out of sequence timing” might seem like elements of a mystery: why would something still have “old” features?

But in fact, if Ventastega is an cousin/offshoot of the branch from which all later land animals are descended, then these terms are really only relative. Ventastega, after branching off from other tetrapods, went its own way: retaining some traits that became modified in the main tetrapod line, and gaining some traits unique to its own branch.

The former retained traits are “primitive” only in the sense that they are more like what earlier tetrapods had than what the particular line of tetrapods that we care most about went on to change. Had those tetrapods gone extinct and Ventastega’s descendants gone on to father all life on land, we’d be talking about the “primitive” traits of the formerly mainline tetrapods, compared to Ventastega.