Archaeopteryx is an icon of evolution and is often called the first bird. Scientists have long debated just how it took off from the ground but a recent study published by my colleagues and I has brought new insights to the mystery.
Our study compared Archaeopteryx with the anatomy of modern birds, and our findings suggest it generated the force needed to take off by taking several jumps forwards.
Archaeopteryx, a fossil uncovered from the Jurassic rocks of Germany, dates back 150 million years. It was covered in feathers and it had wings. But it retained a number of distinctly dinosaur features such as a long bony tail, claws on separate fingers and teeth in a beakless jaw.
Charles Darwin predicted, in the first edition of On the Origin of Species published in 1859, that fossils would later be found linking major groups of animals. A year after publication a feather, and within two years, an entire skeleton of Archaeopteryx had been discovered, showing clear links between birds and the dinosaurs. Archaeopteryx is one of those transitional forms that links one group to another.
The “first bird” was not perfectly developed, as birds have evolved to be today. We would not expect them to be. It had relatively weak flight muscles, with no keeled sternum for strong muscle attachment, and could not have raised its wings above the horizontal.
Rather than call it the first bird, it is better to describe it as the earliest branching species, yet known, on the lineage of birds after the whole bird group split from non-avian dinosaurs some time more than 150 million years ago.
In 2016 my friend Colin Palmer asked a colleague, Markus Heller (professor of biomechanics) and me to join him in exploring a tricky problem: how did early birds slip the bonds of Earth and take to the sky?
Palmer had been working on the flying cousins of dinosaurs, the pterosaurs for over a decade, and also a peculiar (although now one among several known) four-winged, feathered and flying dinosaur called Microraptor. However, pterosaurs are not dinosaurs and Microraptor is a dinosaur closer to Velociraptor (small meat-eating dinosaurs) than to birds.
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There have been a number of suggested mechanisms for the evolution of flight and takeoff, quite literally from the ground up, or the trees down, to flapping while running to climb a slope. The problem is that these are difficult to test experimentally. So we turned instead to the best example of takeoff there is: birds themselves.
Palmer had already filmed a number of birds, out of curiosity. As some other scientists had observed, but had never followed up, Palmer saw that they all showed the same sort of pre-takeoff behaviour. Two leg jumping. The wings appeared to do little in getting the bird off the ground, and this opened up some possibilities for research into the earliest of birds.
The ancestors of birds, the theropod dinosaurs, had powerful legs. They could definitely jump, and jumping could provide an important evolutionary advantage. Being able to get out of the way of a hunter, and maybe adding in a flap of your feathered arms to extend your leap, might just save your life.
The first bit of this research, which we carried out with then undergraduate student Erik Meilak, was developing a means to see how much force the hindlimbs of birds generate in getting off the ground. We used a force plate (a modified kitchen scale) and a Raspberry Pi computer controlling some high-speed photography, along with CT scanning of the skeleton to see exactly how bones were articulated. We worked out how magpies got off the ground. They used their legs as the primary generator.
This undergraduate summer project became a fully-fledged PhD project, with Meilak looking at several birds such as starlings and crows. We worked with evolutionary biologist Pauline Provini in Paris who had done work with doves and finches, demonstrating how all of these birds used their legs in takeoff.
And with detailed kinematics, all of the forces and moments generated by the muscles around the hindlimb joints, we fitted the model to Archaeopteryx and looked to see if the first bird could get into the air in a similar way to modern birds.
Archaeopteryx is by definition 150 million years behind the “leaps” of modern birds, and couldn’t take off as we had observed modern birds do when startled, explosively, with a single jump and then add flapping into the sky. Archaeopteryx couldn’t do that, and as I said couldn’t rely on its wings for powerful flaps. So we had asked what its legs could do. And like birds today, when taking off more leisurely, we demonstrated that Archaeopteryx could have got into the air with two leg leaps. Archaeopteryx could have used three jumps increasing in velocity, and once airborne its flapping wings could have maintained a flight speed of 7 metres per second.
Science Graphic Design, Author provided (no reuse)
However, if Archaeopteryx had added in a flap, it would have got to that speed – jump-flap-jump – with just two leaps. And all this without the highly evolved and flight-adapted anatomy of pigeons, crows, hawks and even chickens.
In the future we may find a fossil that is still a bird rather than a non-avian dinosaur, but is less, well, “birdy” than Archaeopteryx, and then that would become the earliest branching taxon. But we haven’t yet. So Archaeopteryx still sits on its perch as the first bird that we know of.
