You start with fish, swimming around in the ocean. Some of these fish move to rivers and lakes. But life in a river is very different than life in the ocean. First, fish here have to deal with living in fresh water, and then, importantly, they have to deal with changing water levels. Rivers can flood, and they can also dry up. In theory, a fish that could survive drying would have an advantage over those that have can't, and the best way to do that is to figure out how to breathe air.
Of course, once you're breathing air, then why not go all the way, grow some legs and hit the land. After all, it's going to give you a great advantage if you get to land, there are plants and insects there to eat, and predators can't get to you. Voila! life on land.
The fossil evidence supports this approach to settling the land. Take Tiktaalic roseae a transition fossil found by Dr. Neil Shubin and his team in the Canadian Arctic. This skeleton was initially hailed as the missing link between fish and land animals. As Dr. Shubin has described it, Tiktaalic is a fish that can do pushups. That is, it basically looks like a fish with scales, and gills, but its front fins are elongated, and would be able to push off the ground underneath. Think of the movement of seals and sea lions. Not true walking, but a start. Then, it's just a matter of the fins getting stronger, and the back fins elongating, and you're on your way to becoming a four-legged beast.
The timing works too. Tiktaalic lived about 386 million years ago, then about 8 million years later, we find the first evidence of four-legged (tetrapod) life.
But in this week's edition of Nature, a new wrinkle was added to the story. A group of European researchers have discovered a series of trackways (fossilized footprints), of tetrapods of a variety of sizes, from 50cm up to 2.5 m long. They show a wide diversity, and are well enough preserved to show that these are real limbs, with digits at the end. The only problem is that the trackways were made 10 million years before Tiktaalic was supposed to be alive.
That means 2 things. First, our timing of events is wrong. It's likely that Tiktaalic or a relative was the transitional organism to tetrapods. The fact we haven't found older examples of transitional animals isn't necessarily a problem. Fossils are relatively rare, since they only form under specialized circumstances, and it's possible that the transitional animal lived in an area where fossils wouldn't form. The same is true for the earlier tetrapods. Just because we haven't found any fossils doesn't mean they aren't out there.
The second interesting issue raised by this is that we've built a picture of tetrapods developing in rivers and lakes. But these examples are in salt water. Why did they develop there? What advantage did walking on four legs offer in a marine environment? These are new questions that will take more study, and hopefully discovery of more fossils that will answer them.
Nothing like a fundamental scientific shift to start the new year!