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Authors: Jerry A. Coyne
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that.
But we needn’t give up, for we can find some other species in the fossil record, close cousins to the actual “missing link,” that document common ancestry equally well. Let’s take one example. In Darwin’s day, biologists conjectured from anatomical evidence, such as similarities in the structure of hearts and skulls, that birds were closely related to reptiles. They speculated that there must have been a common ancestor that, through a speciation event, produced two lineages, one eventually yielding all modern birds and the other all modern reptiles.
What would this common ancestor have looked like? Our intuition is to say that it would have resembled something halfway between a modern reptile and a modern bird, showing a mixture of features from both types of animal. But this need not be the case, as Darwin clearly saw in The Origin:
I have found it difficult, when looking at any two species, to avoid picturing to myself, forms directly intermediate between them. But this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all of its modified descendants.
Because reptiles appear in the fossil record before birds, we can guess that the common ancestor of birds and reptiles was an ancient reptile, and would have looked like one. We now know that this common ancestor was a dinosaur. Its overall appearance would give few clues that it was indeed a “missing link”—that one lineage of descendants would later give rise to all modern birds, and the other to more dinosaurs. Truly birdlike traits, such as wings and a large breastbone for anchoring the flight muscles, would have evolved only later on the branch leading to birds. And as that lineage itself progressed from reptiles to birds, it sprouted off many species having mixtures of reptilelike and birdlike traits. Some of those species went extinct, while others continued evolving into what are now modern birds. It is to these groups of ancient species, the relatives of species near the branch point, that we must look for evidence of common ancestry.
Showing common ancestry of two groups, then, does not require that we produce fossils of the precise single species that was their common ancestor, or even species on the direct line of descent from an ancestor to descendant. Rather, we need only produce fossils having the types of traits that link two groups together, and, importantly, we must also have the dating evidence showing that those fossils occur at the right time in the geological record. A “transitional species” is not equivalent to “an ancestral species”; it is simply a species showing a mixture of traits from organisms that lived both before and after it. Given the patchiness of the fossil record, finding these forms at the proper times in the record is a sound and realistic goal. In the reptile-to-bird transition, for instance, the transitional forms should look like early reptiles, but with some birdlike traits. And we should find these transitional fossils after reptiles had already evolved, but before modern birds appeared. Further, transitional forms don’t have to be on the direct line of descent from an ancestor to a living descendant—they could be evolutionary cousins that went extinct. As we’ll see, the dinosaurs that gave rise to birds sported feathers, but some feathered dinosaurs continued to persist well after more birdlike creatures had evolved. Those later feathered dinosaurs still provide evidence for evolution, because they tell us something about where birds came from.
The dating and—to some extent—the physical appearance of transitional creatures, then, can be predicted from evolutionary theory. Some of the more recent and dramatic predictions that have been fulfilled involve our own group, the vertebrates.
Onto the Land: From Fish to Amphibians
One of the greatest
But we needn’t give up, for we can find some other species in the fossil record, close cousins to the actual “missing link,” that document common ancestry equally well. Let’s take one example. In Darwin’s day, biologists conjectured from anatomical evidence, such as similarities in the structure of hearts and skulls, that birds were closely related to reptiles. They speculated that there must have been a common ancestor that, through a speciation event, produced two lineages, one eventually yielding all modern birds and the other all modern reptiles.
What would this common ancestor have looked like? Our intuition is to say that it would have resembled something halfway between a modern reptile and a modern bird, showing a mixture of features from both types of animal. But this need not be the case, as Darwin clearly saw in The Origin:
I have found it difficult, when looking at any two species, to avoid picturing to myself, forms directly intermediate between them. But this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all of its modified descendants.
Because reptiles appear in the fossil record before birds, we can guess that the common ancestor of birds and reptiles was an ancient reptile, and would have looked like one. We now know that this common ancestor was a dinosaur. Its overall appearance would give few clues that it was indeed a “missing link”—that one lineage of descendants would later give rise to all modern birds, and the other to more dinosaurs. Truly birdlike traits, such as wings and a large breastbone for anchoring the flight muscles, would have evolved only later on the branch leading to birds. And as that lineage itself progressed from reptiles to birds, it sprouted off many species having mixtures of reptilelike and birdlike traits. Some of those species went extinct, while others continued evolving into what are now modern birds. It is to these groups of ancient species, the relatives of species near the branch point, that we must look for evidence of common ancestry.
Showing common ancestry of two groups, then, does not require that we produce fossils of the precise single species that was their common ancestor, or even species on the direct line of descent from an ancestor to descendant. Rather, we need only produce fossils having the types of traits that link two groups together, and, importantly, we must also have the dating evidence showing that those fossils occur at the right time in the geological record. A “transitional species” is not equivalent to “an ancestral species”; it is simply a species showing a mixture of traits from organisms that lived both before and after it. Given the patchiness of the fossil record, finding these forms at the proper times in the record is a sound and realistic goal. In the reptile-to-bird transition, for instance, the transitional forms should look like early reptiles, but with some birdlike traits. And we should find these transitional fossils after reptiles had already evolved, but before modern birds appeared. Further, transitional forms don’t have to be on the direct line of descent from an ancestor to a living descendant—they could be evolutionary cousins that went extinct. As we’ll see, the dinosaurs that gave rise to birds sported feathers, but some feathered dinosaurs continued to persist well after more birdlike creatures had evolved. Those later feathered dinosaurs still provide evidence for evolution, because they tell us something about where birds came from.
The dating and—to some extent—the physical appearance of transitional creatures, then, can be predicted from evolutionary theory. Some of the more recent and dramatic predictions that have been fulfilled involve our own group, the vertebrates.
Onto the Land: From Fish to Amphibians
One of the greatest
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