A suggested reconstruction of the setting for the pumice-bearing volcanic rock bed. / Cretaceous Res./Elsevier
Where did it all start? Genesis tells of life's beginnings on the Third Day. Darwin pondered whether it happened in a damp pond. And shades of Dr. Frankenstein, scientists tried laboratory lightning, sparking electric arcs through exotic gasses, in a bid to re-create life's earliest moments in glassware decades ago.
Now comes the latest proposal for where life began - on a raft of floating volcanic rock. Pumice, best known as the key ingredient in hard-scrubbing soap, actually floats after erupting from volcanoes. And pumice rafts that sailed the early oceans some 3.46 billion years ago may have served as "cradles of early life," suggests a team headed by paleobiologist Martin Brasier of the University of Oxford.
Brasier thought of the idea while walking on the beaches of Santorini, a volcanic island in the Mediterranean. After a small eruption there, he saw blocks of pumice floating in the sea. "These rafts of rock, rock blocks were just floating offshore," Brasier said. "And it seemed like it may be an answer to some questions about early life."
The questions are how could early biochemistry ever start to flourish more than 3.5 billion years ago, in the early Archean age, when the air was mostly made of volcanic gasses and water vapor, but little oxygen? Studies about the origin of life today generally focus on the transition from chemistry to biology that took place in the world at that time. Early-life researchers such as chemical biologist Jack Szostak of the Howard Hughes Medical Institute see this transition as a long chain of events that saw ordinary chemicals cooking together to make ever-more complex organic chemicals that recombined and essentially competed with one another to proliferate.
Over time, these precursors to the DNA and related stuff comprising our genetic machinery today acquired a covering, or membrane, a primitive cell, where they could complete their chemical reactions in isolation. Last, these "proto-cells" began competing with one another and sharing genetic material in a way that would look like biological life to us today (a gloppy kind of microbial life admittedly, but you have to start somewhere).
"The question I had was where all this early chemistry could take place without being disturbed," says Brasier, whose team reports its latest results in a recent edition of the Precambrian Research journal. "The more I looked at pumice, the more attractive it looked as a place for early life to flourish."
Analysis of the remains of pumice slabs propelled out of the mouths of ancient volcanoes some 3.46 billion years ago and preserved in Western Australia show these little rock islands would have served as perfect Easy-Bake ovens for the earliest biochemistry, Brasier says. These same rock layers have served up some of the earliest controversial evidence of fossil bacteria on Earth. "The pumice there erupted, floated and sank or was beached," Brasier says.
Why does pumice float? Because it has the highest surface-area to volume of any rock. Pumice is puffed full of pores and gas-filled crannies in the volcanic eruptions that create the stuff.
Those pores, some cell-sized, would have served as lab spaces for early biochemical reactions (something not available in tidal pools pounded by waves or near deep-sea hydro-thermal vents exposed to the ocean), the study team proposes. They would have been protected from sunlight fatal to such chemistry in the open. And the suction action of a porous rock bobbing on the ocean surface, or beached and exposed to the tides, would have pulled oily slicks thought to have coated the Achean oceans into those pores, serving up fresh material for reactions deep inside the rock.
In the study analysis, Brasier reports that the Australian pumice was permeated with carbon, oxygen, nitrogen, phosphorus and sulfur, all vital feedstocks for early biochemistry, as well as chemical catalysts such as titanium oxide.
"At this point this is just an idea, but we are reporting that there is some evidence that support it being plausible," Brasier says. A follow-up to the effort, he suggests, might look at the chemistry resulting from the conditions found inside these ancient rocks to see if the right reactions, indeed, would have taken place. "I'm not a chemist, but we think it is worth exploring," Brasier says.
On the skeptical side, "I wouldn't be surprised to see microbial colonization of pumice rafts, but it doesn't seem like a very promising setting for the origin of life, at least to me," says Szostak, a winner of the Nobel Prize in Medicine in 2009. "I think a more complex and diverse environment might be necessary - both chemically and physically. Ponds or lakes in a geothermal active region seem more reasonable to me," he says by e-mail.
Szostak favors ancient freshwater lakes over other proposed settings for life's origin, in part because fresh water seems compatible with primitive cell membranes. Salty ocean water would chew up early protocells, he says. There's also wetting and drying at the edges of ponds to spur biochemical reactions, and perhaps temperature changes from hydrothermal vents like the ones seen today in the modern Lake Yellowstone. In a 2010 publication of the Cold Spring Harbor Perspectives in Biology, Szostak and colleagues ran down some recent experiments suggesting that protocells might form on their own in chemical reactions without needing something like rock pores to serve as a mold for their shape.
At least scientists are thinking about how life started. Darwin himself suggested "a warm little pond" as the setting for life's origin in an 1871 letter to the botanist Joseph Hooker, not too different from Szostak's suggestion. But while Darwin dismissed as "mere rubbish" any attempts to investigate life's origin in his day, it's good to see scientists experimenting and exploring this deepest of questions in ours.
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