New research and discoveries with relevance to astrobiology keep piling up. Here are a couple of the most interesting items from recent weeks.
Raindrops, bubbles hint at a low-pressure early Earth
It's one of those things we often take for granted, but the truth is that we don't really know what the atmospheric pressure on the Earth was 2 or 3 billion years ago. Low pressure can mean less greenhouse effect (a real challenge because the Sun was fainter in the past, yet as far as we know Earth only occasionally suffered from 'snowball' global freeze-downs), it can also change all manner of processes like erosion, water boiling point, and so on.
A paper by Som et al. claims that the sizes of gas bubbles trapped in 2.7 billion-year old lava beds can be a useful proxy for ancient atmospheric pressure. The bubble sizes should run from small to large in going from the bottom to the top of a cooling lava flow (as the pressure of overlaying lava diminishes). But the specific size difference between these bubbles should map to the atmospheric pressure bearing down on top of the lava. Their results seem to broadly corroborate earlier proxy measurements they made from looking at the size of ancient raindrops (which can get bigger and faster in a lower pressure atmosphere).
The bubbles suggest an atmospheric pressure of about 0.23 atmospheres (plus or minus 0.23), while the raindrops suggest a pressure between 0.52 and 1.1 atmospheres. Combined with isotopic evidence the researchers conclude that 2.7 billion years ago Earth's atmosphere may have been only about half of what it is today.
It's intriguing, and could profoundly alter our picture of the early Earth. But these are exceedingly tricky inferences to make. It'll be interesting to watch the geologists fight this one out.
Look Ma! No mitochondria!
One of the key differences between Eukaryotic life and Prokaryotic life is the presence of mitochondria in the former. These structures (organelles) in cells are generally considered to be a form of endosymbiosis from an ancient evolutionary merger of simpler organisms or an acquisition by eukaryotic ancestors. They help animal cells generate far more chemical energy currency than bacteria or archaea. That allows creatures like us to grow big and to express many more genes and develop greater complexity. Mitochondria have also been used as an argument for the extreme improbability of 'complex' life cropping up very often in the universe.
Until now the closest we'd come to spotting a eukaryote without mitochondria were organisms like the Giardia amoeba - harboring what seem to be reduced or remnant mitochondrial structures. These mitochondrial bits don't provide cellular energy like in our cells, but do help Giardia deal with iron-sulfur protein formation.
Now Karnkowska et al. report the discovery of a related single-celled (low-oxygen environment dwelling) eukaryote that has no mitochondrial structures whatsoever. Instead, to carry out the essential iron-sulfur biochemistry it makes use of a system apparently acquired from bacteria.
The assumption is that this organism has lost its mitochondrial pieces over evolutionary time. When, and exactly why, are the next questions for the scientists to pursue. But if otherwise complex-celled organisms can function without mitochondria (albeit in a restricted environment) it does at least raise the question of whether more complex cellular architectures might be less improbable than thought.