We’ve been talking about life quite a bit here recently at Cosmic Variance, and it’s always fun to talk about areas in which one has absolutely no professional expertise. But it’s also fun to bring in experts, which is why we’re happy to welcome Caleb Scharf as a guest blogger. Caleb is Director of Astrobiology at Columbia University, author of a textbook on the subject, an recently jumped into blogging. In this post he reminds us that we’re still learning a lot about the forms of life right here on Earth — knowledge that will be invaluable as we search for it elsewhere.
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It’s a real privilege to be able to write a guest blog for Cosmic Variance and to take a little side trip from my regular postings to Life, Unbounded – the science of origins.
The modern search for life in the universe encompasses everything from exoplanets and astrochemistry to geophysics and paleontology. Underlying and motivating the investigations in these fields – collectively labeled astrobiology – there are some fundamental assumptions, but do they make sense?
In recent weeks one might be forgiven for thinking that a shadowy biosphere surrounds us, aliens are poised to dismantle civilization, and that time traveling species are flitting in and out of view like barflies on a Saturday night. It’s a little disconcerting, does the Kool Aid have something special in it this Spring?
Unfortunately I think that all of these headline grabbing items miss the real story of what life is, here on Earth and potentially further afield. The idea of ‘shadow biospheres’ or multiple origins of terrestrial life sounds intriguing, and certainly helps bring focus to the fact that we can be very blinkered in our outlook. It also steers attention away from a more interesting and demonstrably real point.
In the past couple of decades we have found a shadow biosphere, except that far from lurking in the cracks it turns out to be the biggest, most critical, biosphere on the planet. An astonishing 99.9% of life on Earth cannot be coerced to grow in a lab, and so we have overlooked it. Microbial life – single-celled bacteria and our ancient cousins the Archaea – is not just the stuff under your fingernails, it is what makes multi-cellular life like us function, and it helps govern the grand chemical cycles of our planet, from the continents to the oceans to the atmosphere. Such organisms have, over three to four billion years, evolved into an eye popping array of microscopic machines, the ultimate nano-bots. They can extract energy and raw materials from, it seems, almost any environment. A particularly good example is Desulforudis audaxviator – discovered 2.8 km down in a South African gold mine in a pocket of isolated water. Little audaxviator lives all alone when the vast majority of microbial life is utterly reliant on colonial symbiosis. It earns a living by mopping up the molecular detritus left after radioactive decay in the uranium rich rocks dissociates water and bicarbonates. That’s a very, very neat trick.
Twenty or thirty years ago we barely understood that such life existed on this planet. Now we are beginning to see that the longevity of our biosphere owes itself to precisely this crowd of ‘shadowy’ organisms. A truly wonderful paper was published a couple of years ago in which Falkowski, Fenchel and Delong laid out the big picture for life on Earth. In essence, they argue that single-celled microbial life is the manifestation of an even deeper truth; the core planetary gene set. This is the set of recipes for metabolism, or how to harvest a planet for energy, and we all rely on them. The result of billions of years of natural selection, these genes are widely dispersed across the microbial biosphere. This is true to such an extent that should 99% of life be wiped out by an asteroid collision, supervolcano, or dirty telephone receiver, the information for the molecular machinery that drives all organisms will be safely preserved in the surviving 1%. The living world does not end, it just reboots. Because of this, carbon-based life is a far more robust phenomenon than we could have ever imagined. It is the ultimate, Google-like, cloud computer.
Still though, isn’t this also a blinkered view of what might constitute life? Well, sure, but there’s another fact to consider. When we look out into the universe we find that the chemistry of our life – carbon based molecular structures – is not just occasional, it’s ubiquitous. Carbon is a fabulous player; simple molecules, rings, chains, polymers, sheets, crystals, and great clumps of sooty particles abound. Some is produced directly from the huge outflows of cooling gas from old stars, much forms in the thick nebulae and proto-stellar cocoons that eventually give rise to planets. Thousands of recognizable organic molecules, including amino acids, are found in the treacly mix of some meteorites – the remains of our own ancient solar system. This is a chemical bonanza that must have played a role in setting the stage on the young planet Earth. If this is blinkered then stick a blindfold on me.
So life on Earth is tough and tenacious, and the building blocks are everywhere. Is this enough reason to think that a similar blueprint exists in other places across the universe? Well, it’s definitely motivation to go looking, and to go looking for the kind of exotica that we already know, rather than inventing new ones. Is this reason enough to think that ‘intelligent’ life exists somewhere else? That’s a tough call. Life on Earth did remarkably well for the past 3.5 billion years without us around, I don’t think there is anything that indicates we are more than an evolutionary oddity (albeit an incredible one). It’s a big universe though, with plenty of room for oddities, even if they turn out to be extremely familiar.
Craig Venter and colleagues have achieved a remarkable milestone: they designed a genome, and brought it to life. More specifically, they’ve synthesized a chromosome consisting of over a million DNA base pairs, and implanted it in a bacterial cell to replace the cell’s original genome. That cell then reproduced, giving birth to offspring that only had the synthetic genome. See the Venter Institute press release, discussion in Nature (pdf), more discussion at Edge, and some background from Carl Zimmer. Update: and here is the paper.
