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Paleontologists working in a remote area of the Canadian Arctic have come across a relic of one of our most ambitious ancestors. It's a fish, but one that has taken an important step, literally and figuratively, toward living on land. It's big. It has a mouth packed with vicious teeth, and it didn't need to spend its life underwater.

NPR's Richard Harris tells us more.

RICHARD HARRIS: Evolution takes place everyday in tiny increments. Ted Daeschler at the Academy of Natural Sciences in Philadelphia has been fascinated by how those steps can add up, starting with a creature that swims in the sea and ending up with one that walks on land. That transition, in the lineage that ultimately led to humans, happened about 375 million years ago, long before the dinosaurs.

To look for creatures during that critical time in evolution, Daeschler and his colleagues dug into ancient rock formations in the extreme North of Canada, Ellesmere Island; they poked around for a few years without finding much. But in 2004, they finally came across fossils that caught their attention in a big way.

TED DAESCHLER: We have several individuals. The smallest and best preserved is probably about four feet long. We have the skull and fin of an animal that's significantly bigger, we would estimate at about nine feet long, so these are pretty big fish.

HARRIS: And they were unusual in other ways too.

DAESCHLER: The eyes were placed high on the head almost like a crocodile. They still had scales covering the body, and they still had fins, which you would look at and say, well, that's a fin. It has the long rods that I'm used to seeing in a fish fin.

HARRIS: But this species also had some of the features you'd expect in the earliest four-legged land animals, the tetrapods. For one thing, its head wasn't simply an extension of the body, as you find in a fish. It could pivot from side to side, and the fins had bones that were starting to hint at a hand or a foot.

DESCHLER: The fin was doing much more than a fish that's swimming through open water. In fact, it probably wasn't all that good at swimming through open water.

HARRIS: Instead, this animal was suited to crawling along on its belly through the shallow waters of a muddy river delta. And Deschler argues, this is just the kind of habitat that created an opportunity for fish to emerge from the sea, not with one giant transformative step, but gradually, in fact, over millions of years.

DESCHLER: You don't just swim and you don't just walk on land. There's plenty of ecological space for an animal that did some of each or primarily lived in the water, perhaps on shallow water, using its fins to propel across the bottom or to scoot across a log or perhaps even a mudflat for short distances.

PER AHLBERG: I mean, we don't really know, of course. We're looking at dead bones, and we can only say so much about likely lifestyles.

HARRIS: Per Ahlberg at Uppsala University in Sweden wasn't involved with this find, but he studies the ancient animals, who evolved into land-dwellers.

AHLBERG: There seems a plausible kind of interpretation for the kind of life that this sort of animal might have led.

HARRIS: Ahlberg says the news fossil is quite similar to another ancient species found years ago in Europe, but that similarity actually makes it more interesting scientifically, because it shows that the fossils are no fluke, if you pardon the expression, but part of a sequence that ultimately led to the larger terrestrial animals.

AHLBERG: And so at a stroke, we now get a much more detailed picture of what our own ancestors looked like at the point when they were just starting to nose out of the water.

HARRIS: At the suggestion of Inuit elders who live in the Arctic, Ted Deschler and his colleagues named it ticpolic, which is a word for fish. Its description is published in Nature and for all its significance and evolution, Deschler is careful not to call it the missing link between fish and our early terrestrial ancestors.

DESCHLER: This is a link, it certainly was missing, but there are plenty more links to find in any evolutionary sequence.

HARRIS: And he's anxious to get back to the Arctic again this year to look for more animals on this evolutionary path.

Richard Harris, NPR News.

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