An essay by Richard Corfield in Astrobiology Magazine points up the strange history of the Ediacarans, a group of anatomically diverse organisms that lived during the Ediacaran period (between 635 and 542 million years ago). These creatures, which predated nearly every form of animal life that exists today, stood rooted in bacterial bases on the seafloor, drawing nutrients from the water.

As best we can tell, the Ediacarans lacked mouths and recognizable digestive systems, and their bodies are thought to have looked like "sacks of mud, disks, hubcaps and mattresses." They were among the first complex life forms to appear on the planet, but they bear no discernible resemblance to anything else in the fossil record:

...they have none of the characteristics of the bilaterian animals, which evolved during the Cambrian explosion 542 million years ago. Since then, bilateral animals have provided the basic body plan for every animal that has occupied and dominated the Earth.

The Ediacarans came and went in a remarkably brief interval, geologically speaking. Corfield believes their population exploded when a mass-oxygenation of Earth's oceans coincided with the end of the Cryogenian ice age. Before long, Ediacarans had spread across the globe, as indicated by fossils everywhere from England to Australia. But the rise of mobile, bilaterian animals introduced too much competition to the oceans, and the Ediacarans died off almost as quickly as they'd flourished.

Guy Narbonne, a paleontologist at Queen's University in Ontario, recently told reporters that Ediacaran-descended life could still be with us today, in the form of certain worms and mollusks. For Corfield, though, the interesting thing about Ediacarans isn't whether they still have a presence on Earth, but what their evolutionary arc suggests about whether and how life might develop on other worlds:

the story of the Ediacarans... tells us that evolution can happen very quickly. The idea — first credited to Darwin — that vast amounts of deep time are required for evolution to occur may not be correct. The speed with which the Ediacarans arose in the aftermath of the final Cryogenian glaciation suggests strongly that the evolution of complex, multicellular organisms was on the blocks and just waiting for the starting pistol.

Fossils on the Edge of Forever [Astrobiology Magazine]

Image by Ryan Somma, used under Creative Commons license.

Charles Darwin, recognizing the similarities between humans and chimpanzees, postulated that we might someday find fossils of a "missing link," a creature that represented the evolutionary break between humans and chimps. The discovery of Ardi, however, suggests that when we do find that evolutionary break, the fossils we find will not be a blend of human and chimpanzee.

Researchers discovered Ar. ramidus near Aramis, Ethiopia, and have dated it as 4.4 million years old, considerably older than Lucy, who at 3.2 million years old was considered humanity's oldest relation. It's not clear whether humans are directly descended from this particular hominid, but it makes it clear that bipedal hominids are considerably older than previously thought.

The paleobiologists studying Ardi identify hers as an "intermediate" form, one that is bipedal, but at the same time capable of walking on all forms and traveling through trees. Still, although she represents a point past hominids' evolutionary break with gorillas and chimpanzees, she is very different from modern apes. For example, Ardi's had flat hands and feet and flexible wrists, and engaged in a form of locomotion called palmigrady, which is a trait of ancient apes and unlike gorillas and chimpanzees, which are stiff-wristed knuckle-walkers. This suggests that gorilla and chimp ancestors developed their knuckle-walking long after their evolutionary break with hominids.

In a paper in the upcoming issue of Science, which outlines the discovery, researchers will explain what Ardi's dissimilarity from modern apes means for our picture of human and chimp evolution:

Humans did not evolve from chimpanzees but rather through a series of progenitors starting from a distant common ancestor that once occupied the ancient forests of the African Micoene.

Professor Katsufumi Sato of the University of Tokyo studied five species of soaring birds, measuring the thrust the birds require to stay aloft. He concluded that no animal that weighs more than 40kg (88lbs) could soar. The wandering albatross, which has the largest wingspan of any living bird, weighs only 22kg. Heavier birds can achieve enough thrust to take off, but cannot flap their wings quickly enough to keep their mass in the air. Pterodactyls had as much as a 15 meter wingspan, but were four times heavier than the albatross.

So were pterodactyls the turkeys of the Cretaceous Period? Not according Mike Habib, a biomechanics researcher at the Johns Hopkins School of Medicine:

One possible theory is they would rely on thermals to stay aloft having dropped off the edge of a cliff. However, evidence that they could walked for considerable distances has also been discovered.

Another theory is that their wings were so large that, relatively speaking, wing load was low. If a pterosaur spread its wings, staying on the ground would have been more of a problem than taking off.

So, until paleocloning resolves the question, we can hold onto our vision of pterodactyls in the skies.

Pterodactyls were too heavy to fly, scientist claims [Telegraph]