The first, by a team led by Maxwell Lechte of McGill University, resulted in a paper that appeared in Proceedings of the National Academy of Sciences. Entitled, "Subglacial Meltwater Supported Aerobic Marine Habitats During Snowball Earth," and looked at a curious (and to us, completely inhospitable) time in Earth's history. Current models support the conclusion that for a significant chunk of time in the Precambrian Period, between 720 and 635 million years ago, the entire surface of the Earth was covered with ice. Called the "Snowball Earth" period, it's long been a question in evolutionary biology how any living thing could survive this -- the entire land area of the Earth under a sheet of ice, and the ocean cut off from the atmosphere because its surface is frozen solid.
The authors think they've found the answer. According to their models, subglacial meltwater streaming through stress cracks in the ice would have been sufficient to generate oxygen-rich "oases" in which life could have survive the deep freeze. The authors write:
The Earth’s most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the “Snowball Earth” glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations."The evidence suggests that although much of the oceans during the deep freeze would have been uninhabitable due to a lack of oxygen, in areas where the grounded ice sheet begins to float there was a critical supply of oxygenated meltwater," said study lead author Maxwell Lechte in a press release. "This trend can be explained by what we call a ‘glacial oxygen pump’; air bubbles trapped in the glacial ice are released into the water as it melts, enriching it with oxygen... The fact that the global freeze occurred before the evolution of complex animals suggests a link between Snowball Earth and animal evolution. These harsh conditions could have stimulated their diversification into more complex forms."
The second study is of a very peculiar species of bacteria, Metallosphaera sedula, which is from a curious group of microbes called chemolithotrophs -- they "eat rocks" as part of their required metabolism. Some chemolithotrophs break down minerals like pyrite (iron sulfide), but Metallosphaera is even weirder than that. It requires minerals -- more specifically, the elements in those minerals -- found in significant quantities only in meteorites.
Metallosphaera sedula [Image by T. Milojevic et al.]
"Meteorite-fitness seems to be more beneficial for this ancient microorganism than a diet on terrestrial mineral sources," said lead author Milojevic in a press release in Science Alert. "Our investigations validate the ability of M. sedula to perform the biotransformation of meteorite minerals, unravel microbial fingerprints left on meteorite material, and provide the next step towards an understanding of meteorite biogeochemistry."
Besides that, it also brings up a couple of interesting questions -- first, it immediately made me wonder about the largely-ignored idea of panspermia -- that the earliest life on Earth came here from elsewhere in the universe. The objection has always been that it'd have to be a pretty hardy life form to survive both both the vacuum of interstellar space and the fiery descent and collision of the meteorite with Earth's surface. The Milojevic et al. study suggests that the first part might be entirely possible -- if the earliest life forms were chemolithotrophs, there's no reason they couldn't have been out there on a piece of space rock, nestled in a crack and chowing down on the minerals.
The other question, though, it the extent to which we're doing the reverse -- bringing terrestrial microbes out into space, contaminating every world we visit. The conventional wisdom always was that the trip through space would effectively destroy any microorganisms riding on the outside of the spacecraft, but Metallosphaera sedula shows that might be more of an issue than we thought.
In any case, it does show that life is a great deal more resilient than we ever dreamed, further bolstering my contention that it's common out there in the universe. The so-called "Goldilocks Zone," in which there are Earth-like conditions that foster the generation of life, might be a great deal larger than we ever dreamed.
Long-time readers of Skeptophilia have probably read enough of my rants about creationism and the other flavors of evolution-denial that they're sick unto death of the subject, but if you're up for one more excursion into this, I have a book that is a must-read.
British evolutionary biologist Richard Dawkins has made a name for himself both as an outspoken atheist and as a champion for the evolutionary model, and it is in this latter capacity that he wrote the brilliant The Greatest Show on Earth. Here, he presents the evidence for evolution in lucid prose easily accessible to the layperson, and one by one demolishes the "arguments" (if you can dignify them by that name) that you find in places like the infamous Answers in Genesis.
If you're someone who wants more ammunition for your own defense of the topic, or you want to find out why the scientists believe all that stuff about natural selection, or you're a creationist yourself and (to your credit) want to find out what the other side is saying, this book is about the best introduction to the logic of the evolutionary model I've ever read. My focus in biology was evolution and population genetics, so you'd think all this stuff would be old hat to me, but I found something new to savor on virtually every page. I cannot recommend this book highly enough!
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]
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