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Was The Humble Sponge Earth's First Animal?

By Ben Harder
for National Geographic News
April 1, 2002
 
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.

In his laboratory at Woods Hole, Sogin and several colleagues compared genetic sequences from numerous marine organisms. In the analysis, they studied several species of sponges, sea anemones, and jellyfish, as well as an assortment of more complex animals such as mollusks and echinoderms. They also included examples of fungi and single-celled, choanocyte-like organisms called choanoflagellids, neither of which belong to the animal kingdom.

Sogin extracted genetic material called RNA from the various organisms and compared two types of genetic data from each. The two types of RNA told slightly different versions of the family history of the animals, but both sets of evidence agreed on many accounts.

Once deciphered, the genetic clues revealed that, of all animals, sponges are the most genetically distinct. Jellyfish and anemones share slightly more genetic similarities with each other and with other animals.

This finding led Sogin to conclude that sponges occupy the oldest and lowest branch on the animal family tree. Because the higher branches have introduced additional innovations that account for animals' rich diversity, he says, the common ancestor of all animals probably resembled modern sponges much more closely than anything else alive today.

Although sponges have changed little over the past 500 million or more years, Sogin is quick to note that no living sponge possesses the exact genes or form of the animal Eve. "Evolution is not a process that stops," he says.

A Surprising Link

That sponges are an essential part of our evolutionary heritage is a startling realization for many, but Sogin reached an even greater revelation when he looked slightly farther back on the animal lineage.

"The special evolutionary relationship between animals and fungi was a big surprise," Sogin says. "In many regards, fungi are similar to primitive plants." Yet the fungus, he has concluded, shares "a unique, common evolutionary history with the animal."

Some details of early animal evolution still remain to be worked out. In particular, Sogin would like to know whether certain types of fungi are more closely related to animals than other types. If they are, it would mean that the entire animal family is just a branch on the evolutionary tree of the fungi. In a sense, people—and all animals—would be highly evolved fungi.

Another unanswered question is how some of the early branches of the animal family tree fit together. After sponges, Sogin thinks, jellyfish evolved, and then anemones, which gave rise to the first animal with bilateral symmetry.

Once the step toward symmetry was made, animal evolution appears to have quickly gained momentum.

But the precise order of those first awkward steps toward today's complex array of animal life, and what triggered evolution's accelerating pace, Sogin says, "is still a big mystery."

This story is featured in The Shape of Life, an eight-part television series produced by Sea Studios Foundation for National Geographic Television and Film in association with PBS.

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