This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we discuss the origins of birds, consider one of the longest-known and most famous “transitional forms” – the stem-group bird Archaeopteryx – and examine more recent discoveries that have greatly illuminated a century-old mystery.
From this amphibian-like origin, crown-group tetrapods continued to diversify and acquire new characteristics that we can mention only briefly – such as the key transition to laying eggs on land with a membrane separating the egg from the terrestrial environment (i.e. an amniotic egg), and the subsequent diversification of amniotes into reptiles, birds, and mammals (as well as a number of extinct lineages that are known only from the fossil record).
Archaeopteryx lithographica, a stem-group bird exhibiting features transitional between non-avian theropod dinosaurs and crown-group birds. [source:Wikimedia Commons]
Just winging it
With all of this tetrapod diversity to explore, we can only hope to visit a few points of special interest along the way – transitions that will also serve to illustrate other features of evolution that we have not yet discussed in great detail.
One transition that has long fascinated scientists is the origin of birds, with their striking adaptations of feathers and powered flight. These adaptations place modern birds at a sizeable distance from other present-day tetrapods – a fact that was troublesome to Darwin. Only a few years after the publication of his On the Origin of Species, however, a stunning stem-group bird was discovered in Germany – Archaeopteryx lithographica.
Even to a casual observer, Archaeopteryx displays a mix of “reptilian” and “avian” characteristics. Like reptiles, Archaeopteryx had a long bony tail, teeth, and forelimbs with clawed digits – but combined these features with a trait that until then had been thought to be the sole hallmark of birds – feathered wings. For Darwin, Archaeopteryx was simultaneously support for his theory as well as a reminder of the paucity of the fossil record. He would express these thoughts in a letter to a colleague in 1863:
The fossil Bird with the long tail & fingers to its wings (I hear from Falconer that Owen has not done the work well) is by far the greatest prodigy of recent times. It is a grand case for me; as no group was so isolated as Birds; & it shows how little we know what lived during former times.
Let us now look to the mutual affinities of extinct and living species. They all fall into a few grand classes; and this fact is at once explained on the principle of descent. The more ancient any form is, the more, as a general rule, it differs from living forms. But, as Buckland long ago remarked, all extinct species can be classed either in still existing groups, or between them. That the extinct forms of life help to fill up the intervals between existing genera, families, and orders, cannot be disputed. For if we confine our attention either to the living or to the extinct alone, the series is far less perfect than if we combine both into one general system. With respect to the vertebrata, whole pages could be filled with illustrations from Owen, showing how extinct animals fall in between existing groups… Another distinguished palæontologist, M. Gaudry, shows that very many of the fossil mammals discovered by him in Attica connect in the plainest manner existing genera. Even the wide interval between birds and reptiles has been shown by Professor Huxley to be partially bridged over in the most unexpected manner, by, on the one hand, the ostrich and extinct Archeopteryx (sic), and on the other hand, the Compsognathus, one of the Dinosaurians—that group which includes the most gigantic of all terrestrial reptiles.
So, even in Darwin’s time, the evidence supported the hypothesis that Archaeopteryx was a transitional form in the sense that we have been discussing – as a stem group on the lineage leading to modern birds – with that stem rooted within theropod dinosaurs.
Theropods of a feather, group together
Despite the early discovery of Archaeopteryx, other fossil species that “fill up the interval” between crown-group birds and extinct theropods were unknown until over 100 years later. In the mid 1990s, however, the first of what would be a number of significant discoveries was made in deposits of the Yixian Formation in China – namely, a feathered, non-avian theropod dinosaur named Sinosauropteryx prima. This fossil was noteworthy not merely because it was feathered and related to Compsognathus, but also because of the nature of its feathers – this theropod possessed only relatively simple “protofeathers” – unbranched filaments that could serve as insulation. Sinosauropteryx seems to have branched off the avian lineage at a time when feathers were not (yet) even branched, let alone adapted for flight.
An artist’s interpretation of Sinosauropteryx prima, a non-avian theropod with primitive, filamentous feathers. Coloration in feathered dinosaurs can be inferred from the shape of cells (melanosomes) responsible for feather pigmentation. [source: Wikimedia Commons]
Not long after the discovery of Sinosauropteryx, an additional “transitional form” – i.e. stem group on the avian lineage displaying transitional characteristics – was found in China that revealed a progression in feather evolution towards the feathers found in modern birds. Sinornithosaurus is not only a feathered, non-avian theropod, but one with tufted and branched feathers, which up until its discovery, had only been observed in birds:
These discoveries (and many others – feathered dinosaurs don’t even seem to make the popular press anymore, since they are now so common) thus “fill in the space” between non-avian theropods and birds – to the point where the boundary is so blurred that some new discoveries are debated as being “true birds” or merely very close non-avian relatives.
The use of “half a wing”
So, despite decades of protests from antievolutionists that no possible intermediate forms for birds could exist, we see a group of fossil species that meets the criteria handily – and demonstrates that yes, there was a use for “partially-formed feathers and wings.” Long before birds took to the air, feathers were a common feature of terrestrial, non-flying dinosaurs with a body plan markedly similar to birds. Indeed, at this time and place in biological prehistory, the body plan of “non-avian, feathered theropod” (and later, “quasi-avian, feathered theropod”) seems to have been a remarkably successful one, as evidenced by the number of species we observe in the fossil record that fit this general description. Feathers and wings are thus examples of exaptation– the repurposing of parts through evolution. Feathers were originally selected for a non-flight function (insulation, for example) and later were co-opted for another function (flight). Likewise, the wing is a repurposed (i.e. exapted) tetrapod forelimb. Had Darwin lived to see their discovery, no doubt he would have seen feathered theropods as another “grand case” for his theory.
In the next post in this series, we’ll return to the tetrapod lineage leading to our own species – that of mammals.