When scientists from Virginia Tech and the Chinese Academy of Sciences recently studied three bizarre spherical fossils that date back from 600 million years ago, they were perplexed: analysis of the fossils indicated that they were multicellular lifeforms.
However, this is contradictory to what scientists currently believe: that single-celled organisms did not evolve into multicellular beings until just about 60 million years ago, much later than the period of history that these fossils came from.
"This opens up a new door for us to shine some light on the timing and evolutionary steps that were taken by multicellular organisms that would eventually go on to dominate the Earth in a very visible way," says Shuhai Xiao, professor of geobiology in the Virginia Tech College of Science. "Fossils similar to these have been interpreted as bacteria, single-cell eukaryotes, algae, and transitional forms related to modern animals such as sponges, sea anemones, or bilaterally symmetrical animals."
Scientists originally found the fossils inside phosphorite rocks found in the Doushantuo Formation in southern China. Xiao previously studied these fossils in 1998 and even thought then that they could represent animal embryos.
In this research, the team cut the rocks into thin slices and then used light to look inside the fossils with microscopes, which revealed multiple cells, separated into clusters. The cells had different shapes and sizes, suggesting that different cells had different functions, including some that involve reproductive processes. This differentiation is a distinct feature of multicellular animals.
Researchers believe that the fossils, called Megasphaera, could show the evolution of single-celled organisms into multicellular ones. This would make these fossils the earliest animals ever to walk the Earth.
However, the specific anatomy preserved in the fossils also suggests that they could be from a more plant-like organism, such as algae.
"The bottom line is that they are multicelled and that they have cellular differentiation and that they have separation of reproductive cells from sterile somatic cells," says Xiao. "This is a big thing, because if you look at modern multicellular organisms, including animals, this is a critical step towards very complex multicellular organisms."
The research team hopes they will find more fossils like this, including cells that may have been embryonic. The goal is to recreate the life cycle of the fossils. If the team's original analysis holds true, that these fossils represent multicellular animals, then that radically changes what we know about the evolutionary timeline.
