Speaker 1: Welcome to brain Stuff production of iHeart Radio. Hey brain Stuff,
Speaker 1: Lorn Vogel Bomb. Here out in the Indian Ocean, about
Speaker 1: two hundred and fifty miles or four hundred kilometers to
Speaker 1: the northwest of Madagascar, there's a shallow lagoon encircled by
Speaker 1: a ring of islands. Those outcrops make up the Aldabra Atoll,
Speaker 1: a place where mangroves flourish and one hundred thousand giant
Speaker 1: tortoises roam free. Recently, a different resident caught the world's attention.
Speaker 1: The Aldabra rail is a chicken sized bird found exclusively
Speaker 1: on the atoll. It's also the only remaining island bird
Speaker 1: in the Indian Ocean that happens to be flightless. Weak
Speaker 1: arm muscles and asymmetrical flight feathers keep the bird grounded,
Speaker 1: yet its ancestors could fly. The Aldabra rail evolved from
Speaker 1: the white throated rail, a still living bird that flies
Speaker 1: very well. Thank you. White throated rails inhabit Madagascar and
Speaker 1: neighboring islands. Thousands of years ago, a number of these
Speaker 1: birds flew out to the Aldabra Atoll. Then, as now,
Speaker 1: large predators were rare on the atoll, but the thread
Speaker 1: of predation mostly gone, the bird's descendants gradually lost the
Speaker 1: ability to fly. That same thing happened to the dodo,
Speaker 1: another island dwelling bird whose ancestors surrendered flight. Flying is
Speaker 1: a high energy activity. When there's no need to fly
Speaker 1: away from predators and you can get food simply by
Speaker 1: walking around, why waste the energy on the Aldabra toll.
Speaker 1: Flight became unnecessary for short term survival, So over many generations,
Speaker 1: the isolated rail population gave rise to the fully flightless
Speaker 1: birds we know today. But it turns out there's a
Speaker 1: startling plot twist. Apparently the sequence of events we just
Speaker 1: described happened more than once. A twenty nineteen studies suggests
Speaker 1: that flighted colonizing rails came to Aldabra and begot a
Speaker 1: non flying subspecies on two different occasions. It's as if
Speaker 1: natural selection hit the reset button. Scientists call the phenomenon
Speaker 1: iterative evolution. Today, we're going to explain what this process
Speaker 1: entails and what it doesn't. University of Partsmith biologists Julian P.
Speaker 1: Hume and David Martil co authored the groundbreaking new study,
Speaker 1: which appeared in the Zoological Journal of the Linean Society.
Speaker 1: Since their paper was published, Hume and Martell's work has
Speaker 1: garnered a lot of press coverage. Unfortunately, their findings have
Speaker 1: been widely misinterpreted. To hear, some media outlets tell it
Speaker 1: the modern Aldabra rail went extinct and then resurrected itself
Speaker 1: from the dead. But that's not what happened, and it's
Speaker 1: not how iterative evolution works. Photographers love the Aldabra Toll
Speaker 1: for its sunny beaches and blue lagoon. If you're a paleontologist,
Speaker 1: the islands have another draw a bountiful fossil record going
Speaker 1: back hundreds of thousands of years. On elate piccard the
Speaker 1: westernmost island, a dig site has yielded a pair of
Speaker 1: fossilized arm bones from prehistoric rails. Geologic clues tell us
Speaker 1: the bones are more than a hundred and thirty six
Speaker 1: thousand years old. It looks like the dead birds could
Speaker 1: have used a good flood insurance policy. Judging by the
Speaker 1: distribution of marine fossils like oceanic mollusk remains, it appears
Speaker 1: the atoll was totally submerged under water multiple times in
Speaker 1: the past four hundred thousand years. More we sently the
Speaker 1: islands disappeared beneath the waves were about a hundred and
Speaker 1: thirty six thousand to a hundred and eighteen thousand years
Speaker 1: ago due to a rise in sea levels. Afterward, the
Speaker 1: waters were treated and the atoll re emerged. And now
Speaker 1: here's where the story takes an unexpected turn. The elip
Speaker 1: card arm bones look almost identical to the ones we
Speaker 1: see in living Aldabra rails today, which, as you'll recall,
Speaker 1: are flightless. Therefore, the birds those fossils belonged to probably
Speaker 1: couldn't fly either. So theoretically, when the atoll flooded, the
Speaker 1: prehistoric rails in question were unable to escape and got
Speaker 1: wiped out. However, the saga didn't end there. As human
Speaker 1: Martel explain in their paper, the fossilized footbone of a
Speaker 1: much more recent rail was once extracted from Grand Terror,
Speaker 1: another island in the atoll. That specimen is only about
Speaker 1: a hundred thousand years of age ergo its owner lived
Speaker 1: after the sea levels went back down and the algebra
Speaker 1: atoll resurfaced. In an intriguing case of deja vous, this
Speaker 1: fossil closely resembles the bow in today's non flying Aldabra
Speaker 1: rail and the assumption rail a bird that went extinct
Speaker 1: in ninety seven. A primary sources indicate that it was
Speaker 1: flightless too, but chances are the grand Terra fossil came
Speaker 1: from a bird that either couldn't fly or was in
Speaker 1: the process of losing its ability to do so. Either way,
Speaker 1: it was the probable ancestor of modern aldebra rails. According
Speaker 1: to human Martil, we're looking at an evolutionary do over.
Speaker 1: The flightless birds that died out when the atoll went
Speaker 1: under had descended from an ancestral stock of high soaring rails.
Speaker 1: Once the islands vanished and then re emerged, those aerial
Speaker 1: wanderers repopulated the atoll and evolved into an all new
Speaker 1: flightless subspecies, one that's still at large today. History repeated
Speaker 1: itself loud and clear, and that's iterative evolution and a nutshell.
Speaker 1: Iterative evolution can be defined as the repeated evolution of
Speaker 1: a specific trait or body plan from the same ancestral
Speaker 1: lineage at different points in time. Let's say there's an
Speaker 1: organism or a closely related group of organisms with a
Speaker 1: fairly conservative build that manages to survive over a long
Speaker 1: period of geologic time. If multiple groups of similar looking
Speaker 1: descendants independently evolved one after another from this common ancestor.
Speaker 1: It would be a clear cut case of iterative evolution.
Speaker 1: Consider the ammonites, spiral shelled relatives of squids and nautilus is.
Speaker 1: Ammonites roamed the oceans throughout the age of dinosaurs. Some
Speaker 1: experts think that individuals with thinner shells that were compressed
Speaker 1: from side to side were better suited for shallow environments
Speaker 1: with very fast currents. On the other hand, thicker, heavier
Speaker 1: shells nicely lent themselves too deep areas far off shore,
Speaker 1: So there's evidence that in certain parts of the world,
Speaker 1: an ancestral stock of thick shelled ammonites would periodically give
Speaker 1: rise to thin shelled descendants who invaded beachside habitats. When
Speaker 1: the sea levels fell, many of those habitats disappeared and
Speaker 1: the offshoot ammonites died out. But their thick shelled ancestors persisted,
Speaker 1: and when the oceans rose again, they'd sire a new
Speaker 1: generation of shallow water denizens with thin shells. And that's
Speaker 1: just one example. Innerative evolution might also explain the repeated
Speaker 1: rise and fall of similar looking sea cows over the
Speaker 1: past twenty six million years. Likewise, sea turtles, specifically the
Speaker 1: ones with seagrass centered diets, may have undergone the same
Speaker 1: process during their evolutionary history. While natural selection is a
Speaker 1: powerful force, cannot revive an extinct species, but when the
Speaker 1: environmental conditions are right, you can at least produce a
Speaker 1: good imitation. Today's episode was written by Mark Vancini and
Speaker 1: produced by Tyler Clang. Brain Stuff is a production of
Speaker 1: iHeart Radio's How Stuff Works. For more in this lots
Speaker 1: of other topics topics topics, visit our home planet, how
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