Draw a line back through time from today’s person, panda, porpoise, pelican, or perch and it ought to end with their earliest progenitor. In the mists of the ancient past, a single organism must have given rise to us all. But that raises an interesting question: Where did this animal come from? What did it look like? And what are its nearest living relatives?
To understand what the first animals looked like, Mitchell Sogin, an evolutionary microbiologist at the Marine Biological Laboratory in Woods Hole, Massachusetts, used advanced automated DNA technology and computing power to trace the molecular evolution of dozens of today’s oldest known species—jellyfish, sea anemones, sponges, mollusks, starfish—back to their common point of origin. When he grouped the species in the precise order of their appearance on Earth, from less complex to more complex, he landed on sponges.
Common throughout the British Isles, the Scypha ciliata sponge has a hairy, fingerlke body that may grow to about one inch in length but is often as small as one or two centimeters. It is normally found along the shore, under rock overhangs or attached to seaweed, but can live in deeper waters. Water and nutrients pass through millions of tiny pores into a central cavity and exit through an osculum, a large opening (left) at one end. A “crown” of spicules (center and right) surrounds the osculum. The spicules are composed of minerals—a crystalline form of calcium carbonate—and apparently help protect the sponge.
All SEM photomicrographs courtesy of Louise Kerr and Gavin Histen/Marine Biological Laboratory, Central Microscopy Facility at Woods Hole.
Even Sogin was taken aback. “Sponges didn’t seem like animals—they didn’t go seeking prey, didn’t have 4 legs—or 10 legs. Show Joe Blow a sponge and it looks like cauliflower. But it’s not. It’s an animal.” Perhaps even more intriguing, Sogin uncovered something older in the animal line than sponges that isn’t an animal: fungi. “That’s surprising,” Sogin says. His findings have implications for evolutionary studies and may even shed light on the shape of extraterrestrial life. The discoveries have already made contributions to medicine. “He’s a pioneer in the systematic application of this method,” says evolutionary biologist W. Ford Doolittle of Dalhousie University in Nova Scotia. “It’s a very great achievement.”
Not bad for a Chicago kid who “never expected to become a scientist” and in fact had “no driving career ambitions” when he went to school. Now, sitting in his cluttered Woods Hole office, the soft-spoken biologist says, “It seems to be the big questions that appeal to me.” He opened his lab in 1989 with 5 people (now 10) and eight years later founded the Josephine Bay Paul Center for Comparative Molecular Biology, which he directs. Both are funded by the National Institutes of Health, the National Science Foundation, and NASA. He gazes out the window at icy Eel Pond and Buzzard’s Bay beyond it, then throws a wistful glance at a photo of Woods Hole in summer, when he sails his wife’s 41-foot Beneteau sloop, Origins.
Sogin and his colleagues are examining basic questions not only in linear molecular evolution but also in molecular ecology, molecular biodiversity, the evolution of genomes, and parasitology. Questions posed decades ago by Carl Woese, his mentor at the University of Illinois, and other scientists—such as how the essential unit of life, the cell, came into being—are still unanswered. Sogin says, “I’m obsessed with finding our origins—where we come from and where we are.”
There are 9,000 species growing up to eight feet tall, from the tropics to the Arctic. They don’t visibly move or stalk prey or appear to mate; they just sit there as the world’s oceans pass through their pores, filtering as little as an ounce of food from a ton of seawater. Many even live in freshwater. Sponges are multicellular, but the cells don’t add up to much: no tissues, muscles, organs, nerves, or brain. But this simplicity can be deceptive. Some sponges come armed with glasslike skeletal spikes, microscopic and as beautiful as snowflakes. Some, like the fire sponge of Hawaii, have surface toxins that can cause excruciating pain to humans—and in which scientists have begun to discover antitumor and anti-inflammatory agents.
The sponge is the earliest, most primitive multicelled animal, Sogin says. Some scientists believe the ability to grow different cell types started animals on the evolutionary road to becoming humans. With just a few kinds of cells, only loosely connected, the sponge manages to produce a variety of asymmetrical shapes, from cups and fans to tubes and piecrust shapes. Sponges survive handsomely on their own and can even shelter other sea creatures: Scientists found a large sponge in the Gulf of Mexico hosting 16,000 snapping shrimp and 1,000 other aquatic animals. The sponge’s cells, its calcium carbonate or glasslike silica spicules, and the mass of collagen that forms its visible body all create a network of tunnels and chambers, with little flailing hairs called cilia on the walls that wave the water through and filter out plankton and waste. No matter how large the sponge, it can eat only what its individual cells can absorb.
Sponges are also the earliest sexual reproducers; most are hermaphroditic, producing both eggs and sperm, which they release into the water. The sperm drift along until they find their way into the tunnels and caves of another sponge. But the sponge has other reproductive options. If you push one through a sieve, breaking free its individual cells, these cells will drift until they find each other, then stick together and create an exact genetic duplicate of the parent. If wounded, a sponge doesn’t need to grow new tissue; it simply moves old cells into the wound to close it. These techniques have helped sponges survive at least 500 million years. A few have remarkable capabilities. One, living in a Mediterranean underwater cave, traps small crustaceans with the sharp, glassy spikes jutting from its body, then surrounds them with its cells and digests them.
Biologist Calhoun Bond, then at the University of North Carolina at Chapel Hill, found in 1986 that sponges don’t just sit still—many actually move. Using time-lapse microscopy, he filmed freshwater sponges slowly crawling across the bottom of their containers. He found that larger, saltwater sponges do the same by extruding flat paddlelike extensions of their bodies and pulling themselves along, often climbing the sides of their glass tanks in labs. One sponge, a lavender beauty called Heliclona loosanoffi, moved four millimeters a day.
With so many curious characteristics, sponges have always been hard for taxonomists to place. Although some biologists have suspected they are more animal than plant, others have considered them to be outside the evolutionary line that led to multicelled metazoans—today’s animals. Until recently, neither side had a good way to prove its case.