Nature's first sexual encounter took place among tubular invertebrates called Funisia dorothea, which lived about 565 million years ago, an archaeological study suggests.
Paleontologists found the F. dorothea fossils in 2005 on an ancient seafloor in the South Australian outback.
The ropelike creatures were tightly packed into groups that resemble those of modern sponges and corals.
These living invertebrates use a reproductive technique that releases floating eggs and sperm to produce mass births of many offspring, called larval spatfalls.
Funisia dorothea seen branching in a fossil excavated in South Australia [Image: Droser lab, UC Riverside]
In a paper that appeared last week in the journal Science, the researchers argue that the way the F. dorothea fossils were found suggests they might have used the same body positions to ensure sexual success.
"We can't say 'definitely' about something that happened 565 million years ago," said Mary Droser, study co-author and professor of earth sciences at the University of California, Riverside.
"But it's very likely that this was sexual reproduction."
F. dorothea are part of the so-called Ediacaran biota, the first multicellular life-forms to evolve beyond bacteria, plankton, and algae.
The intriguing animals were common from about 580 million years ago until the start of the Cambrian Explosion about 540 million years ago, when the fossil record began to include the lineages of almost all living animals.
Scientists stress that most of the Ediacaran biota are not very well understood.
"These things are wacky looking," Droser said. "They don't look like anything we see today and we don't have a clue what most of them are."
Bruce Runnegar, a paleontologist at the University of California, Los Angeles, isn't quite convinced that F. dorothea would have been capable of sex.
Researchers still can't be certain whether the ancient organisms were truly animals, how they reproduced, or when sex first arose within the line leading to modern animals, he noted.
"It's a fine paper and it sorts out a puzzling [set of fossils] that has been known since the 1960s," Runnegar said.
"[But] I would say that the case for this being evidence for the 'first-ever example of animal sex' is weak."
Study author Droser notes that individual F. dorothea fossils also used an asexual reproduction technique called budding, which is employed by modern sponges and yeast.
The evidence that the ancient critters might have reproduced in multiple ways suggests to her that their early ecosystem was, in some ways, surprisingly advanced.
"It means that some complex survival strategies still in use by today's invertebrates were used in truly ancient times," Droser said.
"[Different reproductive techniques] are not just evolutionary adaptations to threats such as predators and competition for food resources," she added.
"It's sort of amazing that these earliest animals on the planet were rather complex, and that Earth's first ecosystems had rather complex ecologies."
By Brian Handwerk