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A tiny organism that lived a billion years ago had two different cell types, one forming its core and another its outer 鈥渟kin鈥�. It may have been one of the first life forms built that way, making it a crucial step towards modern organisms like animals that also have a skin that is distinct from the cells inside the body.

鈥淭his fossil clearly is multicellular with two different types of cell,鈥� says Charles Wellman at the University of Sheffield, UK. While organisms made of multiple cells were known to have existed for hundreds of millions of years, he says, having such an ancient one with recognisable cell types is new.

The first organisms were single-celled 鈥� as many still are today 鈥� but, through the process of evolution, some began joining up to form larger multicellular organisms.

鈥淚t鈥檚 actually surprisingly common,鈥� says Emily Mitchell at the University of Cambridge. Multicellularity evolved independently several times in different groups. Eventually, some of these multicellular groups became large and complex organisms such as animals and plants.

The oldest hard evidence of animals is from around 600 million years ago. Yet multicellular red algae have been reported from 1.6 billion years ago. There are even reports of multicellular organisms as early as 2.1 billion years ago. While not all these reports are convincing, says Mitchell, it seems clear that multicellular organisms remained simple for a long time.

Wellman and his colleagues studied microfossils preserved in rocks in north-west Scotland. The rocks are about 1 billion years old and formed in an ancient freshwater lake.

Within the rocks, there are fossil remains of a new species, which the researchers have named Bicellum brasieri. Each B. brasieri was a clump of a few dozen cells a few tens of micrometres across. There was a central ball of tightly packed oval cells, surrounded by an outer layer of sausage-shaped cells.

Some of the fossils only had the central ball. The team suggests these show the organism in a larval stage. 鈥淭he cells keep dividing to form this ball of cells, and then different cells form in it that elongate and these appear to migrate to the outside and form an outer layer,鈥� says Wellman.

The team believes the two cell types might be explained by basic physics. The late biologist Malcolm Steinberg argued that if some cells stick together better than others, that will be enough to generate quite complex structures, an idea he called the . For Wellman, the structure of B. brasieri is compatible with this idea.

B. brasieri wasn鈥檛 a true animal, emphasises Wellman. 鈥淭here鈥檚 a long, long way to go until you get real animals,鈥� he says. But it probably belonged to the larger group from which animals arose. 鈥淚t鈥檚 telling us about the really early events in that lineage.鈥�

鈥淪tudies like this show how some of the early steps towards complex multicellularity may have occurred,鈥� says Jennifer Hoyal Cuthill at the University of Essex, UK. 鈥淕roups of cells with two distinct types may have been a key step on the way to the many structurally differentiated cells and, later, tissues that enable animal-level complexity.鈥�

For Mitchell, the 鈥渕illion-dollar question鈥� is why it took multicellular organisms hundreds of millions of years to give rise to complex animals like sponges, jellies and worms. For many years, it was assumed that an increase in oxygen levels around 550 million years ago enabled the rise of animals, but in recent years, evidence has emerged that ancient animal groups like sponges can survive at low oxygen levels 鈥� and that the oxygen level only rose after the first animals had appeared.

鈥淚t couldn鈥檛 have been the perturbation in the atmosphere that caused animals,鈥� says Mitchell, so there must be another explanation.

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