In Ridley Scott's 1979 slimy monster masterpiece, Alien, the extraterrestrial life form discovered by Sigourney Weaver and crew goes through two startlingly different phases after it hatches. Is such a change during the life of an animal mere sci-fi license?

Not really. In fact, many Earthlings go through similar drastic changes in form. Think, for example, of the caterpillar and butterfly, or the tadpole and adult frog.

This Australian sea urchin larva is one organism studied for clues to what aliens might be like. After several weeks of feeding, this will metamorphose into a juvenile sea urchin. Credit: Rudolf A. Raff

"Is there anything that we can learn that would allow you to make any predictions about life elsewhere? I think there is, even if the genetic systems aren't the same. There are going to be rules that you suspect are going to apply across life in many places. Some of those are based deep in chemistry. But at the other end, you might suspect that a lot of life history features are going to be the same. I think that there may be lessons that we learn that are not strictly earthbound in any sense."

These lessons are painted with a broad brush, cautions Bruce Runnegar, a professor of paleontology, and the principal investigator for the University of California, Los Angeles, NASA Astrobiology Institute lead team.

"I don't think we'll find out what extraterrestrial life looks like," he said. "I don't think you can say that we'll have something that'll have four legs or fur." Instead, Runnegar said, the study of life on Earth will lay out some basics of how life might have evolved on another planet, but few specifics.

Raff agrees. Understanding evolution and development won't tell us what protects alien skin (Earth organisms use slime, scales, feathers and fur) or how many legs an extraterrestrial life form might have. There are just too many accidents of evolution on Earth. For example, terrestrial Earth mammals walk on four limbs not because it's the perfect way to support a body, but because they inherited four limbs from their four-finned ancestors, lobe-finned fish.

But the better we understand the interactions between development and evolution -- how development evolves and how it constrains evolution -- Raff argues, the better we will be able to know what to look for as we search for signs of life outside our biosphere.

"I think that there's just an enormous wealth of considerations, as to what you would expect an alien to be like, that we begin to learn about through these kinds of studies of evolution and development," Raff said.

Raff studies sea urchins, organisms that go through a change of form as drastic as that of the monster in Alien, albeit much less scary. Sea urchins hatch from eggs as microscopic floating larvae. These delicate, elegant-looking creatures have two mirror-image halves, just like beetles, birds and bats. They float for weeks near the ocean's surface fanning even tinier organisms into their mouths.

Then the change occurs. A small bundle of cells inside the organism begins to grow. All of the other cells die and the tiny bundle settles to the ocean floor to grow into something that looks like a calcified Koosh Ball™, several inches in diameter. Instead of two mirror-image halves (bilateral symmetry), the adult urchin is organized more like five equal pie slices (pentaradial symmetry), a nearly spherical version of its cousin the starfish. Adult urchins creep slowly around the ocean bottom using a five-toothed structure to scrape food from rocks.

So here's an organism that hatches from an egg as a bilaterally symmetrical larva and becomes a pentaradial adult. "It's clear from both morphological [body shape] and particularly molecular studies that their ancestors were bilateral," Raff said. "Here's this big transformation of body plan. So the real question is: How did it happen?"