In less than five milliseconds, a Hydromantes salamander can launch its tongue—including the muscles, cartilage, and part of its skeleton—out of its mouth to snag a hapless insect mid-flight. Among amphibians, it is the quick draw champ. Frogs and chameleons are comparative slowpokes when it comes to their ballistic anatomies. “I’ve spent maybe 50 years studying the evolution of tongues in salamanders,” says David Wake, an evolutionary biologist at the University of California, Berkeley, “this is a particularly interesting case because salamanders, who don’t do anything fast, have the fastest vertebrate movement I’m aware of.” Within their lineage, evolution found a better way to accomplish tongue-hunting. Their seemingly unique adaptation appears to have evolved independently in three other unrelated salamander species. It is a case of convergent evolution—where different species separately developed similar biological adaptations when faced with the same environmental pressures. Salamanders are Wake’s go-to example when asked a decades-old question in evolutionary biology: If you could replay the “tape of life” would evolution repeat itself? In the salamanders, it appears it has: In other organisms, it may not have.
This question was famously posed by the late evolutionary biologist Stephen Jay Gould in his 1989 book, Wonderful Life: The Burgess Shale and the Nature of History, which was published at a time when people still listened to music recorded on cassette tapes. The book discussed fossils left behind by myriad strange animals that inhabited the Earth’s oceans about 520 million years ago during the Cambrian period, and were preserved in the Burgess Shale. Nearly all animals alive today can trace their lineages back to the creatures that lived in the Cambrian, but not every animal that lived in the Cambrian period has descendants that live today. Many Cambrian species have since died out because they weren’t fit enough to compete, or because they were in the wrong place at the wrong time during volcanic eruptions, asteroid impacts, or other extinction events.
Gould saw the incredible diversity of the Burgess animals and theorized that life today would have been different had history unfurled in another way. Random mutations and chance extinctions—events Gould called “historical contingencies”—would build on each other, he suggested, driving the evolution of life down one path or another. In Gould’s view, the existence of every animal, including humans, was a rare event that would have been unlikely to re-occur if the tape of life were rewound to the Cambrian period and played again. One of the paleontologists—Simon Conway Morris of Cambridge University—whose work on the Burgess fossils was heavily cited by Gould in his book, strongly disagrees with this viewpoint.
Conway Morris believes that, over time, natural selection leads organisms to evolve a limited number of adaptations to the finite number of ecological niches on Earth. This causes unrelated organisms to gradually converge on similar body designs. “Organisms have to configure themselves to the realities of the physical, chemical, and also biological world,” he says. In Conway Morris’s view, these constraints make it all but inevitable that if the tape of life were replayed, evolution would eventually reproduce organisms similar to what we have today. If humans’ ape ancestors had not evolved big brains and the intelligence that goes with them, he believes that another branch of animals, such as dolphins or crows, might have, and filled the niche that we now occupy. Gould disagreed.
Both scholars recognized that convergence and contingency exist in evolution. Their debate instead revolved around how repeatable or unique key adaptations, like human intelligence, are. Meanwhile, other biologists have taken up the puzzle, and shown how convergence and contingency interact. Understanding the interplay of these two forces could reveal whether every living thing is the result of a several-billion-year-long chain of lucky chances, or whether we all—salamanders and humans alike—are as inevitable as death and taxes.
Rather than attempt to reconstruct history with fossils, Richard Lenski, an evolutionary biologist at Michigan State University, decided to watch convergence and contingency unfold in real time, in the controlled environment of his laboratory. In 1988, he separated a single population of Escherichia coli bacteria into 12 separate flasks containing liquid nutrients, and let them each evolve separately. Every few months for the past 26 years, he or one of his students has frozen a sample of the bacteria. This archive of frozen microbes gives Lenski the ability to replay E. coli’s tape of life from any point he wishes, simply by thawing out the samples. Along the way, he can examine how the bacteria change both genetically and in ways that are visible under a microscope. Lenski says, “The whole experiment was set up to test how reproducible evolution was.”
This question was famously posed by the late evolutionary biologist Stephen Jay Gould in his 1989 book, Wonderful Life: The Burgess Shale and the Nature of History, which was published at a time when people still listened to music recorded on cassette tapes. The book discussed fossils left behind by myriad strange animals that inhabited the Earth’s oceans about 520 million years ago during the Cambrian period, and were preserved in the Burgess Shale. Nearly all animals alive today can trace their lineages back to the creatures that lived in the Cambrian, but not every animal that lived in the Cambrian period has descendants that live today. Many Cambrian species have since died out because they weren’t fit enough to compete, or because they were in the wrong place at the wrong time during volcanic eruptions, asteroid impacts, or other extinction events.Gould saw the incredible diversity of the Burgess animals and theorized that life today would have been different had history unfurled in another way. Random mutations and chance extinctions—events Gould called “historical contingencies”—would build on each other, he suggested, driving the evolution of life down one path or another. In Gould’s view, the existence of every animal, including humans, was a rare event that would have been unlikely to re-occur if the tape of life were rewound to the Cambrian period and played again. One of the paleontologists—Simon Conway Morris of Cambridge University—whose work on the Burgess fossils was heavily cited by Gould in his book, strongly disagrees with this viewpoint.
Conway Morris believes that, over time, natural selection leads organisms to evolve a limited number of adaptations to the finite number of ecological niches on Earth. This causes unrelated organisms to gradually converge on similar body designs. “Organisms have to configure themselves to the realities of the physical, chemical, and also biological world,” he says. In Conway Morris’s view, these constraints make it all but inevitable that if the tape of life were replayed, evolution would eventually reproduce organisms similar to what we have today. If humans’ ape ancestors had not evolved big brains and the intelligence that goes with them, he believes that another branch of animals, such as dolphins or crows, might have, and filled the niche that we now occupy. Gould disagreed.
Both scholars recognized that convergence and contingency exist in evolution. Their debate instead revolved around how repeatable or unique key adaptations, like human intelligence, are. Meanwhile, other biologists have taken up the puzzle, and shown how convergence and contingency interact. Understanding the interplay of these two forces could reveal whether every living thing is the result of a several-billion-year-long chain of lucky chances, or whether we all—salamanders and humans alike—are as inevitable as death and taxes.
Rather than attempt to reconstruct history with fossils, Richard Lenski, an evolutionary biologist at Michigan State University, decided to watch convergence and contingency unfold in real time, in the controlled environment of his laboratory. In 1988, he separated a single population of Escherichia coli bacteria into 12 separate flasks containing liquid nutrients, and let them each evolve separately. Every few months for the past 26 years, he or one of his students has frozen a sample of the bacteria. This archive of frozen microbes gives Lenski the ability to replay E. coli’s tape of life from any point he wishes, simply by thawing out the samples. Along the way, he can examine how the bacteria change both genetically and in ways that are visible under a microscope. Lenski says, “The whole experiment was set up to test how reproducible evolution was.”
by Zach Zorich, Nautilus | Read more:
Image: Daniel Zender
