The Oldest Animal Fossils Discovered to Date Were Alive About 635 Million Years Ago

More than 635 million years ago, sea sponges dominated the Earth's oceans. Scientists have made the discovery based on steroid-type substances that distinguish the existence of the "oldest known fossil evidence of animal life" The National Geographic quotes University of California, Riverside researcher Gordon Love.

The discovery is based on organic chemicals found in sedimentary rock deposits that were analyzed by the Oman national oil company in an oil field located within the Arabian Peninsula in South Omar. The significance of these findings is that they establish the existence of "multicellular animal life on Earth 100 million years earlier than previously believed -- well before the so-called Cambrian explosion 545 million years ago, when such complex organisms were thought to have begun evolving and proliferating dramatically."

"The chemicals, steroids distantly related to testosterone and estrogen, are a unique marker for sponges."

The animals' remains don't look like traditional fossils. They're more like fossil echoes: chemical traces of a compound only produced — at least in modern times — by demosponges, descendants of what some scientists consider to be the last common ancestor of all animals.

"Based on chemical signatures inside sedimentary rocks, Gordon Love and colleagues think the sponges likely grew in colonies that blanketed areas of the ocean floor.

"Back then the supercontinent Rodinia, which had been Earth's dominant landmass for at least 350 million years," Love explained, "was in the process of breaking up, and the climate was extremely cold worldwide.

"It is, definitively, the earliest evidence for animals," said geochemist Gordon Love, who was the main author of the study published in Nature.

"Love's team identified the fossils whose ... sediments date to the last stages of the the aptly-named Cryogenian period after a deep freeze referred to by scientists as Snowball Earth.

"Sponges evolved in shallow ocean basins, because the deeper seas did not yet contain oxygen, a necessity for almost all life."

"Although the environment was harsh at this time," The National Geographic explained, "about a hundred million years before the evolutionary growth spurt known as the Cambrian explosion—a lack of predators made life easier for the sponges."

"There was no competition from more complicated animals, so sponges were probably thriving," Love concluded. "Compared with other times in our history, there were enormously high amounts of them."

"Love and colleagues were able to date the sea sponges because the animals' chemical traces were found in rocks beneath glacial deposits from an ice age that ended about 635 million years ago."

The scientists cut away the outer surfaces of the rock, cleaned the remaining core with solvents, and crushed what was left behind into a powder that could be chemically separated into its component parts.

"It just so happens that these sponges produce very distinctive chemical structures," explained Love, whose team describes their results in this week's issue of the journal Nature.

"Their presence in the Oman rock shows that these precursors of bathtub sponges were the dominant species on the planet for as long as 100 million years, researchers reported Thursday in the journal Nature."

According to the Los Angeles Times: "Sponges are one of the simplest multicellular organisms alive. They live on shallow sea floors and eat detritus that drifts from above. The walls of their cells contain steroids called 24-isopropylcholestanes that are not present in other species."

"After four decades of research, they have never been seen in unicellular organisms other than in trace amounts," said research scientist Gordon Love."

Love and his research teams findings demonstrate that: "Multicellular life far preceded the 'Cambrian explosion' 545 million years ago."

"Until now, the oldest animal fossils dated to Earth's next geological period," called the Ediacaran; "Also known as the Vendian period, a geological period that began at the end of the last ice age of the Snowball Earth. The Edicaran period lasted from 640 million years ago to the start of the Cambrian period 543 million years ago. During it, the first multicellular life-forms, including sponges, jellyfish, and worms appeared. The Ediacaran is named after the Ediacara Hills, just west of the Flinders Ranges in South Australia, where peculiar fossils were found in the 1940s. Later it was realized that examples had been reported earlier on other continents.." Scientists had been unsure whether they reflected the actual birth of animal life, or merely the beginning of the fossil record.

"The new findings show that animals indeed evolved before the Ediacaran, giving these humble sponges at least 100 million years to develop the kaleidoscopic physiologies that bloomed during the early Cambrian period."

"Biologists might argue about which animals diverged first," said Love, "but regardless of that, we're certainly looking at very basal animals."

"Kevin Peterson of Dartmouth College and his colleagues had independently hypothesized that sponges lived about 650 million years ago based on biological clues in the genes of modern sponges."

"To see to a robust, geochemical record of a tremendous amount of sponge mass at this time is very exciting," said Peterson, who was not involved in the new study."At some point during this interval, sponges gave rise to more complex organisms, including eventually vertebrates," he said.

"The origin of complex life is rooted in sponge biology, and that's what makes it so exciting for us."

"Other recent research suggests that an ancestor of placozoa — an amoebalike creature whose genome was sequenced in 2008, providing genetic clues of an ancient lineage — was the first animal. Regardless of this taxonomical controversy, however, Love's fossils are clearly old. Both uranium dating and the fossils' sedimentary position confirm a late-Cryogenian origin."

"In a commentary accompanying the findings paleobiologists Jochen Brocks and Nicholas Butterfield raise the possibility that some other organism than a sponge may have left the 24-isopropylcholestane behind. Fossil descendents of sponges, they note, have not been found in the Ediacaran or Cambrian periods."

"Love, however, called the chemical a paleobiological smoking gun."

"Screening has been done on modern organisms, and there's only one that produces these in abundance: demosponges," he said. "One day, we might come across a microbe, but it hasn't happened so far."

"Love next plans to further excavate Cryogenian sediments in order to determine exactly where and when his proto-sponges developed."

"Was it the conditions of the first glaciation that caused a change in biology?" Love questioned. "Was it the aftermath, during a change in ocean chemistry? We're trying to understand the context of the first appearance of animals."