Viruses lurk at the hairy edge of what we generally consider to be 'living' things. Small infectious structures, they replicate only by digging into a host's intracellular environment and hitching a ride. The smallest are barely 10 nanometers across - the size of a wavelength of ultraviolet light - and coded for by only a few thousand nucleotide base-pairs (compared to the more than 3 billion that code a human). Vast numbers of viruses exist in nature. In marine environments there can be 250 million individuals per milliliter of water, most of them so-called bacteriophages - targeting microbial hosts. Viruses play fast and loose with genetic material, clipping, swapping, dropping and incorporating at a mind-boggling rate. There is no doubt that they have played a critical role in the molecular evolution of life on Earth over the past 4 billion or so years. Our human genome contains millions of fragments of viral DNA - accumulated by our distant, distant ancestors and ourselves. Viruses bring into question even the very notion of 'species', we are all molecular ragdolls, a button sewn on here, a piece of thread incorporated there.
A newly discovered virus steps up the ante (and adds to the zoo of such things). Infecting a single-celled marine organism known as Cafeteria roenbergensis (good name, apparently it's a voracious eater) this virus is a monster. Its genetic makeup is 730,000 base-pairs, with 500 regions that look like bona-fide genes, many likely involved in making protein structures - something most viruses don't bother with. Like all viruses it's not a cellular organism, but here it is doing much of what regular living things do. It not only seems to have genes that could help make cell membranes, it even seems to have stolen genes directly from bacteria.
The line between this remarkable structure and 'life' is thin indeed and I think makes it very clear that there is more of a continuum than any dramatic 'jump' between complex molecules and what we clearly recognize as organisms. We have barely scratched the surface in our understanding of the relentless activity of the microscopic world. Intriguingly this Megavirus infects one of the major predators of the marine microbial environment. In that context it's hitching a ride with an organism that hoovers up both bacteria and viruses as food. What an excellent smorgasbord of genetic material it must see, one cannot help but wonder if that opportunity has helped it build its own massive library.
In the quest for life in the cosmos, and particularly as we continue to poke around in our own solar system, we need to think carefully about what we are looking for. Fragments of DNA from a formerly aqueous environment on Mars - should they ever be found - would likely offer a window into this borderworld between molecules and self-contained organisms. It's going to be messy.
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6 comments:
I think it is a bit misleading to think of viruses as a stage between non-living complex molecules and live organisms. They are degenerate organisms, i.e. they have ancestors that were full-fledged life forms in their time. They just slid into a niche where they could afford to lose most of their own genes because their host organism provided the goods.
I think there really is a wide gulf between the dead and the living, and the link between them is tenuous and unique, lost in the distant past.
Careful about that Carter paper. It is not peer-reviewed, googling "vatches" yields only one topical match, which goes back to the same person. The juxtaposition of vatches and Alzheimers is just weird, and in my somewhat informed opinion it reads much like pseudoscientific rambling.
I can't vouch for its wrongness, but it is definitely not mainstream.
Re the comment about viruses having full fledged ancestral organisms - I'm not familiar with that idea, what's the basis?
Well, a simple one is that of logic:
Every virus needs a host, so there could not have been viruses before there were hosts. This applies to the first ever virus. Barring a second abiogenesis, that first virus must have ancestors, which must be organisms.
The various theories of viral evolution are summarized nicely here: http://en.wikipedia.org/wiki/Virus#Origins
They may not exactly match my one-sentence initial claim, but I think they all make viruses a sideline, rather than a critical link.
My view as expressed earlier would coincide with the first, the second is also plausible. I am not sure about the third, it seems the closest to what you had in mind.
That said, some features of viruses seem like they might be relevant for the evolution of life. The crystalline protein coat as an alternative to lipid bilayers for confinement is interesting in this respect. So are RNA viruses or viroids that do not encode proteins. They could conceivably be remnants of (or regressions to) the "RNA world", the deeply mysterious time before proteins and the genetic code.
In my view the regression hypothesis for viruses is the most likely, because we see regressions everywhere around us. Parasites and endosymbionts all are regressed organisms, i.e. they gave up their independence for the "easy life", and then lost much of the complexity needed to maintain that independence.
So, I think you are right that viruses are a path between life and non-life, but it is a one-way street, like an alpine ski slope. It goes from life to non-life, and does not necessarily provide any clues about the opposite direction. That opposite direction is likely to be completely different, like a ski lift.
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