Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Friday, July 2, 2010

Original multicellular life

We are cellular clumps. Great big, sticky, masses of about ten trillion cells, plus our microbial passengers. Although they all operate in unison, our cells are each quite self-contained. Membranes hold together our 3.3 billion base-pairs of information packaged up tight, and all manner of other molecular structures that allow the cell to hum away like a tiny machine. On the other end of the spectrum are the microbes, where an entire living organism is encapsulated in one single cell.

Somewhere in-between is a fascinating and tricky regime. Most microbial life - bacteria and the ancient archaea - live in colonies, typically of many, many different species. They communicate, they glue together in biofilms, they both support and rely upon each other. It is so very tempting to look at this and see how life on Earth went from these single-celled characters to multi-cellular grandeur. It's just a matter of joining the dots - literally.

Fossil evidence for when and how multi-cellular life on Earth first reared up has been awfully tough to come by though. Small squishy things do not take well to billions of years of mineralization. It does seem to have coincided rather broadly with the time at which Earth's atmosphere began to load up with significant amounts of oxygen about 2.4 billion years ago - courtesy of photosynthetic organisms like cyanobacteria. Available oxygen, while toxic to much of microbial life, opens up a whole new, energetically favorable, chemical network - a big tipping point towards large and complex lifeforms. A fabulous paper appeared this week in Nature, by El Abani and collaborators, along with a nice opinion piece by Donoghue & Antcliffe, describing perhaps the closest thing yet to our multi-cellular ancestors - hot on the tail of the oxygenating Earth.

In this paper they report fossil remains of curious looking, bumpy, swirly, scallopy characters as big as 12 centimeters, from what is now Gabon. At 2.1 billion years old they land on the cusp of atmospheric transformation. Carbon and sulfur isotopes seal the deal on these being biological in origin (see my other post). Everything about these objects screams cellular colony. Does that make them multi-cellular? Well, incredibly, the patterns in these fossils strongly suggest that the type of cell-to-cell communication that is the hallmark of creatures like ourselves was at play.

So here we are, meet your great-to-the-power-of-nine-aunt. In the search for life elsewhere in the universe we often come up against the question of whether 'complex', multicellular life is going to be rare or not. Certainly its longevity on planets probably comes down to fine details of environmental stability. However, if microbial life can gentle slither into this mode of operation - like our friends from Gabon seem to be doing - I think it ups the odds that for at least part of a biosphere's history, multi-cellular type life will be around. Perhaps we should think of life like us more as a desert bloom - it may happen only briefly when conditions are right, but the potential is always there.

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