To discover the fossils of organisms from close to when complex life first started to come together and form multicellular structures is a rare occasion. Not least because these first organisms lacked many features we’d recognize in modern life forms, but also because soft tissue preservation is in itself very rare, even more so when the rocks being studied are over 500 million years old.

Now, researchers have described not just one, but two new species of multicellular organisms from this elusive period.

This was all in the build up to the Cambrian explosion around 540 million years ago, where life suddenly radiated quickly and diversified, giving rise to most major animal groups. But before all that, life was much simpler. It was during the Ediacaran period when the first definite multicellular life starts to appear in the fossil record, around 600 million years ago. With hard shells yet to evolve, evidence of creatures from this time is scant and hard to interpret. Most animals were probably anchored to one spot, with segmented worm-like bodies or feathered fronds.

^{The researchers describe two new species, Chinggiskhaania bifurcata (a-h) and Zuunartsphyton delicatum (i-j). Dornbos et al. 2016}

The fossil of one of the most well-known animals, Dickinsonia, looks quite simply like a ribbed oval bag. What these life forms looked like while alive is much trickier to discern. Certain types of rock are more likely to preserve these soft-bodied creatures, but the formation of these rocks, called Burgess Shale-type deposits, from the Ediacaran are incredibly rare. It was after stumbling across one such formation in Mongolia that researchers found the fossils of two multicellular marine algae organisms – what we would know today as seaweed – which date to an impressive 555 million years ago.

“This discovery helps tell us more about life in a period that is relatively undocumented,” explains Stephen Dornbos, first author on the paper published in Scientific Reports, in a statement. “It can help us correlate the changes in life forms with what we know about the Earth’s ancient environments. It is a major evolutionary step toward life as we know it today.”

The organisms found by Dornbos and his colleagues might not look like much by today’s standards – simple strands twisting across the rocks – but they fill an important gap in our understanding of the environment in which multicellular life appeared.