Monday, March 12, 2012

"Walking with Tetrapods"

Here is an interesting video from Nature about a radical discovery of fossil footprints that are believed to predate the time at which the current fossil evidence suggests was the first walking-on-land event.

One of the elusive questions in evolutionary biology is when did animals first walk on land? Before this discovery, the fossil record (which includes body fossils and preserved trackways) of the earliest tetrapods dated the water-to-land transition at the Late Devonian period. However, the tetrapod tracks found in Poland suggest that animals first walked the land in the early Middle Devonian period - nearly 20 million years earlier than paleontologists had believed based on early tetrapod body fossils. 

The stride length, relative spacing of the footprints, and absence of body drag (which would be seen if the creature was more fish-like), demonstrate that the pathway was made by tetrapod locomotion.

 The fossilized footprint has clear impressions of short, triangular toes.

As explained in the video, the trackways clearly belong to a four-legged creature - there are obvious imprints of a walking pattern indicative of an animal with forelimbs and hind limbs. Also, a closer look at the individual footprints reveals the distinct impressions of separate digits (toes) and footpads. This novel discovery is now forcing scientists to reevaluate the timeline of the fish-tetrapod transition and reassess the conditions under which our very distant ancestors moved out of the water and onto land, taking their first steps.

1. Nied┼║wiedzki G, Szrek P, Narkiewicz K, Narkiewicz M, Ahlberg PE. Tetrapod trackways from the early Middle Devonian period of Poland. Nature. 2010;463(7277):43–48.


  1. Why tetrapods? Are there any organisms that are tripods? Four seems like a reasonable number because of the symmetry, but why not other numbers? Insects have 6 limbs- so does this automatically mean they are evolutionarily more "advanced" than tetrapods?

    1. I think it's likely that there are two different reasons that explain "why one number, not another". One could be that there are selective constraints, so that a tripod body would not be able to survive in the environment (lower fitness). Another involves developmental constraints. This could be because mutations leading to a certain body plan can cause problems in the development of other areas, or because too many mutations are required to obtain the configuration. I'm guessing they both play a role in why, for example, there are no tripod mammals (to my knowledge)

      Also, evolution is not like Pokemon. There are no levels. (But tarantulas and cephalopods are definitely at the top.)