New Microfossils Flesh Out the Story of Multicellular Life

Ancient embryo-like fossils have been newly discovered; these pristine microfossils provide evidence that similar aged microfossils might also be traces of the earliest animals and not large bacteria as previously suggested.

One of the 600 million year old multi-celled microfossils. Photo: Virginia Tech

One of the 600 million year old multi-celled microfossils. Photo: Virginia Tech

Life has been swarming on Earth for 3.5 billion years, yet only in the last billion years did these single-celled characters get together to form sophisticated multi-celled entities. Single-celled life is tiny, microscopic in fact, while multicellular life is essentially every other thing you could imagine: apes, moss, mosquitoes, frogs, tape worms.

Having more than one cell has its obvious advantages, but there are many factors that must first evolve before life can become multicellular. For single cells to team up and form a multi-celled Megazord, they must evolve the capacity to adhere to each other, they must assume different roles where necessary, and finally, they must evolve to commit the ultimate sacrifice – individual cell death for the greater good. This feat occurred independently in the beginning of plant, animal, and fungus history.

Megazord, a single entity with committed individuals as legs, arms, torso, and head. Al Pavangkanan/Flickr/CC BY 2.0

Megazord, a single entity with committed individuals as legs, arms, torso, and head. Al Pavangkanan/Flickr/CC BY 2.0

As you might imagine, finding fossils to study the evolution of multicellularity is not easy. The earliest examples of multi-celled creatures were less than a millimetre across and existed hundreds of millions of years ago. Try spotting those in a slab of rock.

Fortunately, there’s a spectacular rock formation in China where scientists do just that. It’s called the Doushantuo Formation and it is a special place for paleobiology. The Doushantuo rock was formed half a billion years ago when the sand beds of lagoons and sea shores became compacted into unique rock known as phosphorite. When the microscopic life died in these salty waters, the structures of their bodies were preserved. Thanks in part to the phosphate-bearing sea salt, these tiny creatures were eventually preserved as microfossils in the Doushantuo phosphorite, now analysed by researchers.

Many microfossils have been found in this formation. At first, these microfossils were interpreted as multi-celled creatures, possibly the first amassing of cells that would go on to become plants, animals, or fungi – the groups we know and love.

Researchers also suggested that these microfossils might represent life further along the ‘evolutionary tree’, they may be the ‘eggs’ or ‘embryos’ of sponges or other early animals. If these hypotheses were correct, such structures might signal the earliest examples of multicellularity or even true animals, exciting findings indeed.

Subsequent studies were then published suggesting that these microfossils were not multi-celled life, but bundles of single-celled organisms such as algae, protozoa, or even giant bacteria.  The identity of these microfossils was now up in the air: were they an early example of multicellularity, or just another stock standard cell – the type that had been swimming around for billions of years?

Then came the current study, a geobiologist from Virginia Tech, along with collaborators from the Chinese Academy of Sciences, analysed overlooked slabs from the Doushantuo Formation and found exactly what they were looking for: three dimensional microfossils preserved with such clarity that their identity was unequivocal. They were clearly multi-celled organisms, potentially even embryonic animals.

The phosphorite had preserved the cellular integrity of each cell in the multi-celled organism, allowing the researchers to observe many telling details. The fossils had many cells, adhering to each other like Megazord’s arms, legs, and torso. Some cells were specialised for reproduction, while some were not. And finally, the microfossils showed evidence of programmed cell death, that special cellular sacrifice – ‘one for all’.

The qualities present in these fossilised organisms represent the transition that plants, animals, and fungi went through so long ago. These features went on to become integral to the construction of all conspicuous life. Without cell adherence, we wouldn’t be a being and we wouldn’t have trees. Without cell specialisation, we would have no brain cells or sperm/egg cells. And without programmed cell death, our hands and feet would be Ping-Pong paddles with bodies incapable of chiselling away the cells fusing our digits.

The researchers will continue to analyse the Doushantuo material. If fossilised adults of the current specimens are found, then these microfossils might be the embryos of some of the earliest animals, all frozen in time. If no adults turn up, then these microfossils might be wild cards – the ancestors to plants, animals, fungi, or a lineage lost to time.

Originally published at Think Inc.

Story source: Science Daily

Research paper: Lei Chen, Shuhai Xiao, Ke Pang, Chuanming Zhou, Xunlai Yuan. Cell differentiation and germ–soma separation in Ediacaran animal embryo-like fossilsNature, 2014; DOI: 10.1038/nature13766