The late Stan Wood, a self-taught paleontologist from Scotland, had an unwavering belief that the earliest animals to walk on land did so in his homeland. In 2011, just a year before he died of cancer, Wood discovered fossils of four-legged animals known as tetrapods at an unassuming place called Willie's Hole, located near Chirnside, Scotland.
The remains dated to between 360 and 345 million years ago, which is when most scientists believe vertebrates-animals with backbones-made the transition from sea to land.
A National Museums Scotland-organized excavation team returned to Willie's Hole in 2015 and unearthed evidence supporting Wood's long-held belief. They found the remains of five new fossil species, which date to about 355 million years ago and are thought to be the earliest known four-legged vertebrates to walk on terra firma. The fossils are described in the latest issue of the journal Nature Ecology & Evolution.
Once on land, tetrapods diverged into two groups: the ancestors of amphibians and the ancestors of reptiles, birds and mammals. This means that in a sense, as mammals, all humans can trace their ancestry back to Scotland.
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Co-author Nick Fraser of National Museums Scotland admits the connection is a colossal stretch, but explained that the first literal animal step on land was "a pivotal step in the evolution of life on Earth." Without this moment, "the evolution of birds, crocodiles, pterosaurs, salamanders, dinosaurs, mammals-and, of course, ourselves-and birds could not have occurred."
What led to that important step, which Fraser likens to the "one giant leap for mankind" first step on the moon, appears to have been the aftermath of the late Devonian mass extinction that occurred 358 million years ago. The land masses that now form Scotland were then in very different locations, closer to the equator.
"We think the area was subject to quite a bit of change-low-lying ponds, lagoons and streams were being subjected to intermittent flooding and occasional inundation by the nearby sea," Fraser said, adding that there were also "periods of aridity."
The conditions were clearly demanding, he continued, so "it might well have been this dynamic changing environment that partly drove the transition to land by some of the early tetrapods."
The move happened gradually, such that the still fishy-looking animals had a semi-aquatic lifestyle, which at least involved returning to water to lay eggs, before later tetrapods evolved to become fully terrestrial. Those animals, "fully emancipated from water," as Fraser said, date to 345 million years ago in Scotland.
This critical time in history has been murky due to a 15-million-year hole in the tetrapod fossil record known as Romer's Gap that extends between 345 and 360 million years ago. Wood's fossils, as well as those of Fraser and his colleagues, help to fill that gap.
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Before the more recent discoveries, some researchers suspected that after the Devonian mass extinction, oxygen levels were low. This environment, they theorized, limited the number and diversity of tetrapods.
Fraser, lead author Jennifer Clack of the University Museum of Zoology Cambridge and their team investigated this theory by examining fossilized charcoal dating to the Romer's Gap time. They determined that atmospheric oxygen levels were stable and did not inhibit the evolution of animal life on land.
The new finds, in fact, suggest that there was a sudden explosion of diverse life during the Romer's Gap period, but Fraser warned that the fossil record can sometimes be misleading due to missing remains and other problems.
"Nevertheless, innovations in the fossil record do often seem to result in rapid bursts of radiation," he added. "Witness the radiation of the dinosaurs at the beginning of the Jurassic, or the mammals once the dinosaurs died out at the end of the Cretaceous."
Top image: An illustration of tetrapods and other animals that lived 355 million years ago in what is now Scotland. Credit: Mark Witton, National Museums Scotland WATCH: Humans Are Speeding Up Evolution