The origin of animals has long perplexed scientists. DNA studies of creatures living today suggest that their common ancestor appeared nearly 800 million years ago, yet the fossil record contains no clear evidence of animals more than 555 million years old. Two new discoveries are starting to resolve that apparent conflict. Together they push the fossil record of animals back another 300 million years.
In a study published in Nature in February, researchers reported finding a steroid compound (called 24-isopropylcholestane) in 675-million-year-old stone cores, drilled from former seabeds up to three miles beneath the deserts of Oman. Sponges are the only organisms known to produce appreciable amounts of this steroid, and geochemist Gordon Love of the University of California at Riverside interprets the chemical signature as evidence that spongelike animals had evolved by then.
Another team reported in Geology in May that they had found meshlike patterns suggestive of sponges in 850-million-year-old rocks. They turned up in an ancient reef built by cyanobacteria, says Fritz Neuweiler of Laval University in Quebec. The earth’s early oceans initially contained little oxygen, but cyanobacteria produce it as a by-product of photosynthesis. “Here we have a local oxygenated environment,” Neuweiler says, “and this would have supported these early animals.”