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Sponges Led the Way

“Chemical fossils” point to first multicellular creatures
April 21, 2009

A week before Charles Darwin’s 200th birthday, and shortly after the close of a conference on campus commemorating his epochal work on natural selection, a key turning point in the history of life on this planet was revealed. An MIT-led team showed that the first multicellular animal life–the lowly sponge–evolved much earlier than anyone had thought.

FIRST SIGNS Evidence of early sponges was found in these sediments in Oman. The oldest sponge steroids were detected below a 635-million-year-old calcium carbonate deposit on rocks of glacial origin, meaning that they predate the “Snowball Earth” event of the Marinoan age.

Even Darwin was puzzled by the apparently sudden appearance in the fossil record, all at once, of a great variety of multicellular creatures–a blossoming known as the Cambrian Explosion–after billions of years in which microbes were the only life. Since Darwin’s time, further fossil studies have pushed back the origin of animal species from the Cambrian, which began 542 million years ago, to sometime during an earlier period known as the Ediacaran, which began 630 million years ago. But the new chemical evidence, reported in Nature, sets the mark much earlier still. The emergence of animal life apparently wasn’t so explosive after all.

Genomic evidence had already led many biologists to believe that sponges–multicellular creatures that feed by passing seawater through internal channels–were our earliest animal ancestors. But soft-bodied animals like sponges are very rarely preserved as fossils, so determining when they originated required some clever detective work. The key turned out to be unusual chemicals: steroids of a type produced abundantly by sponges but virtually never by simpler organisms.

The new evidence, from sediments in Oman, shows that the earliest sponges may predate the Ediacaran by 80 million years. Roger Summons, a professor of geobiology in MIT’s Department of Earth, Atmospheric, and Planetary Sciences who directed the study, says these “chemical fossils” show that sponges, among the simplest forms of multicellular life, were indeed the first such organisms on Earth. “We nailed it by removing all sorts of ambiguities,” ­Summons says. The report was coauthored by Summons’s former postdoc Gordon Love, now at the University of California, Riverside, along with several MIT students and colleagues at other institutions.

“This might also represent the advent of the earliest reef systems made by animals rather than microbes,” Summons says. The establishment of that ecological niche may have helped pave the way for the later proliferation of complex organisms. And that, after several hundred million more years of natural selection, led to the origin of our own species.

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