for National Geographic News
Fish swim. Birds fly. Humans walk, talk, and think. Animals exhibit such an array of diversity in shape and behavior that it's hard to imagine how a single organism could have given rise to them all. Yet Darwinian evolution requires that such an animal once lived.
Mitch Sogin has been doing something humans do so well: He's been thinking. And he thinks he knows how the common ancestor of the animal kingdom—the animal "Eve"—looked.
It looked like a sponge, he says.
For the past decade, Sogin, an evolutionary biologist at the Marine Biological Laboratory in Woods Hole, Massachusetts, has been sifting through clues in the genetic codes of simple marine organisms. The primeval whodunnit he aims to solve is this: Who, or more accurately, what life form, spawned the animal kingdom?
Sponges, some 9,000 species in all, occupy virtually every aquatic habitat on Earth, from freshwater lakes to tropical seas and even Arctic waters.
Although they seem motionless and lifeless to the untrained eye, these organisms are hard workers. A single sponge pumps many gallons of water through its body each day to strain out the tiny, one-celled organisms on which it feeds. Sponges have to filter about a ton of water for each ounce of food they ingest.
For all that impressive activity, however, sponges are simple animals. They possess no nervous system or breathing apparatus, nor do they have limbs or the capacity to move.
The Origin of Complexity
Nevertheless, says Sogin, "the sponge has a lot of organization to it." In particular, it has two different types of cells, each of which plays an important role in the functioning of the whole.
Sponge cells called choanocytes ("coe-ann-oh-sites") each project a minuscule filament. Choanocytes use these filaments, called flagella, to paddle water past themselves.
Thousands of choanocytes beating their flagella in synchrony, like oarsmen on a Roman galley, propel a steady stream of water past the sponge's other cells, which are designed to capture and ingest the food particles the water contains.
Sponges' ability to grow different cell types was an innovation that underlies virtually all subsequent advances in the animal kingdom, Sogin believes.
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