If you are “petrified” of bad science reporting, this weekend probably left you stone-faced. Good ole “Fox News” believed it had uncovered the story of the century: fossilized bacteria are raining on us from outer space! And once it was on Fox, well the internet lapped it up. Bloggers (oh I guess that’s us!) served up opinions by the bucket load.
The idea is interesting: “Panspermia” is the hypothesis that life is transported throughout the Universe on meteroids, asteroids and even planetoids. The perhaps more plausible idea that life could be transported between two nearby planets like Earth and Mars is known as “Transpermia”. The 1996 press release about supposed micro-fossils from a Martian meteorite (they weren’t!) was in this category.
This time around, Richard Hooper, a scientist at Marshall Space Flight Center submitted a paper to the Journal of Astrobiology, a reputable peer-reviewed scientific journal, on microscopic structures that appeared to be like fossilized bacteria. The paper was rejected. However, he found a journal prepared to publish his article, namely the Journal of Cosmology. I won’t go into a long discussion of the problems with this journal (see discussion by Phil Plait) but needless to say it isn’t the most unbiased and reputable place to publish a paper like this.
While many have commented that this is really a problem with the standard of science reporting, the real story here seems to be just how incredibly hard micropaleontology actually is! And when you really think about it, that isn’t surprising.
Macrofossils, generally described as those bigger than 1mm in size, are created in many different ways. The key point is that there are usually many, many cells in the object that can become fossilized, and, even better, porous bone-like structures that can be “filled up”. For objects like this, even if you don’t preserve the inner structure of the cells, you can still preserve the overall structure.
But when it comes to bacteria, you’re talking about single-cell organisms. Indeed the first thought through your head might be “how do they even form fossils?”. Well, it seems to depend entirely upon the bacteria.
One way of forming fossil bacteria is quite straightforward – entrapment in tree resin. Jurassic Park showcased prehistoric flies
encased in “amber”, hardened pine resin, and the same thing can happen to bacteria. But if you want to preserve fossil bacteria in space, this clearly a non-starter – there are no trees on asteroids!
In permineralization, the interior of an organism is “cast” from the inside out at a cell by cell level. This process requires water that carries minerals (silicates are a particularly common example), and the water must also enter the cells – porous cell walls make this easier. Once inside the cell, crystals of the minerals can start to form and eventually may replicate the interior cell structure that they are contained in. Notice that this process doesn’t necessarily do anything to the cell walls themselves.
And that’s why permineralization differs from petrification. In petrification, the organic material of the cell (for example the cell walls) are also replaced by silica. In permineralization you only need to fill the interior of the cell.
But millions of years later, all you are left with is a mineral deposit that looks something like a bacteria. The organic material has long since gone. Once that happens, how sure can we be that the mineral “lumps” are in actually fossilized bacteria? And that inevitably leads to the question, “how easily can you form something that looks like fossil bacteria but isn’t?”
There’s been quite a bit of research on this (here, here, here for examples). The answer: amazingly easy. A lot of attention has focused on the formation of magnetite globules ever since these were argued to be bacterial fossils on the ALH84001 Martian meteorite. On Earth, there are so-called “magnetotactic” bacteria which uptake large amounts of iron and form magnetite deposits within them (and of course become magnetic themselves!).
But given a strong heating event, entirely plausible with any kind of high-speed impact, iron-rich carbonate solutions will precipitate the formation of magnetite globules at fracture points in the rock. The morphology of these objects is a perfect biomimic/morph of fossilized magnetotactic bacteria found on Earth.
There’s also plenty of other work on various self-assembling compounds in silica-rich environments (here, here for example). They even have a name: silica biomorphs. Here’s a nice image depicting the growth of helical structures, and a chart summarizing different morphologies depending on chemical solution pH and crystallization time.. Here’s a link to a fascinating lecture by Juan Manuel Garcia-Ruiz, one of the authors of a highly-cited Science magazine paper on this very topic. His images are startling, because many of the silica-rich microgeological structures look almost exactly like living bacteria.
Of course, morphology alone isn’t the only criteria that helps determine with a structure is or isn’t a fossil bacteria. Trace element analysis to find chemical signatures associated with life already has something to say (see Rosie Redfield’s blog post), and perhaps that will be the key ingredient that finally helps us understand how to separate abiogenic (e.g., geologically created) and biogenic (e.g. created from bacteria) structures.
Despite the difficulty of micropaleontology, panspermia isn’t going away in a hurry. Stayed tuned for a discussion on an upcoming experiment that will shed some light on it.