Tiktaalik

Chris - Now we're winding our watches back 380 million years now to a time when the first animals began to substitute feet for fins and heave themselves out of the sea and onto land, because believe it or not, we all come from the sea. We all evolved there and the complicated animals became early humans and subsequently modern humans all came out of the sea at some point. Now we know that must have happened and fossils from animals undergoing this transition though are really hard to come by. That's what makes this week's discovery really so important. Neil Shubin from the University of Chicago and his colleagues have uncovered Tiktaalik, and this is a fish which has got all of the features of a primitive wrist, and elbow in some of its limbs and it has very bizarre features. It's got a head rather like a crocodile with nostrils on the side and eyes on the top of its head and ribs. Neil told me earlier why his find is so important.

Neil - The transition from a fish that live sin water to a animal that's able to live on land is one the great transitions in the history of life. We know from a variety of different lines of evidence that this transition happened around 380 to 365 million years ago. We also know the likely players involved, the different kinds of fish that are likely to have given rise to land living animals. Now you know that when we're talking about land living animals, we're talking about a whole branch of the tree of life. This is the branch that includes amphibians, reptiles, birds, mammals and even us. So this is part of our own, albeit very ancient, evolutionary past. The discovery's really important because it's the discovery of a new kind of fish that blurs the distinction between fish and land living animals. It has a mosaic of features seen in both. Like a fish, it has scales, fins and a very primitive jaw. But like a land living animal it has ribs that fit together, it has a neck so the head can move around separately, and it has a flat head like primitive amphibians. Importantly, when you look inside the fin, it has a number of bones that compare very closely to the bones in our own wrist, in our own hand. And so in that sense, it's a real mosaic. It tells us to a great extent how parts of our skeleton evolved.

Chris - Where was it found?

Neil - We found it in a place called Ellesmere Island, which is one of the northernmost island in Arctic Canada, 700 miles south of the North pole. When we work there in the summers, it's daylight 24-hours a day. There are polar bears walking around, there are musk ox, and it's a classic Arctic landscape with ice and glaciers. But between the ice and glaciers, there's bedrock and that bedrock is Devonian aged rock of about 375 million years old. The environments that are contained in those rocks reveal an ancient river or delta ecosystem. Within that delta ecosystem we're finding a variety of fish of which this new kind is just one.

Chris - But a common criticism in the past has been that there are hardly any of these fossils ever found, these transitions from one stage to another.

Neil - Exactly, and this really puts a lid on that because not only do we have a beautiful transitional form but we also have multiple skeletons of it. And when you compare this creature to other creatures that we've known about before: a creature called Panderichthys or Elpistostege, which are creatures that are known from the Baltics or Russia; or when we compare them to the earliest land living animals, a creature known as Acanthostega; when you put this whole series together, it's truly one of the most remarkable transitional series between different kinds of life in the history of the Earth.

Chris - How do you think it came by these interesting adaptations in the first place though?

Neil - Let's think about how this thing lived. What it has is a flat head with eyes on top much like a crocodile. It has nostrils on the side. It's a very flat animal. It has an appendage or a fin that's able to bend its elbow and its wrist. It has a rib cage which suggests that it was able to support itself in gravity. What you have is an animal that's clearly specialising for life on the water bottom, in the shallows or even out in the air for periods of time. So if we look at the geology of the site from where this thing came from, it came from a very small stream in a large delta and giant swampy environment. And it's within these small streams that we believe that we find the locus for the evolution of many creatures that later forms used to walk on land.