Thursday, June 29, 2006

Does life shape the landscape?

There are bazillions of stars out there, and untold numbers of planets, some of which are going to harbor life. How do you go about distinguishing those few from the many barren ones? You can narrow the list by thinking about the requirement for liquid water and a reasonably stable star (it turns out those aren't so common) but you've still got a lot of objects to look at.

One possibility is that a living planet will have an obviously different landscape compared to a barren one. On earth, plant roots hold the soil against erosion by rain and wind, which then affects the speed and sediment of rivers, which can then affect the profile of mountain ranges. It is also likely that life transformed earth's atmosphere, possibly several times.

But is there something about earth that would not have occured in the absence of life? A review in Nature last January compared Earth to Mars and came up with suprisingly few definitive differences on the scale of mountains or drainage valleys. What those authors did propose is that although the range of geologic features is similar, the distribution of these features on a biotic planet might be skew detectably relative to an abiotic one.

What struck me as I read the review, though, is that our cousin planets Mars and Venus, both definitely dead at the moment, have REALLY different geology from each other, not to mention Earth. If you saw a similar object around a completely different star, you might be hard pressed to say if it was behaving "normally" (without life) or not.

I was interested, then, to pick up this paper, from the lab of MT Rosing, which proposes that photosynthetic life on earth helped create the surface energy cycle required to form the continents. The basic argument is that plate tectonics requires a lot of energy-- more than the earth's internal heating should generate. However, chlorophyll and company harvest huge amounts of the sun's light, which tilted the whole-earth energy budget in favor of tectonic movement and stable continents (basically by increasing weathering of some rocks to contribute to the tectonic churn.) But this continental drift seems to be a consequence of things which are much easier to detect, like a transformed atmosphere and tons of liquid water.

UPDATE: Molecular biologists, this problem needs you! Check out this primer (subscription, unfortunately) at Current Biology. There's lots to think about.

"Who needs coffee when you have a family of sober organ donors?"

Recent reports suggest that coffee can counteract the effects of alcohol on the liver. The Onion, of course, has the definitive reaction.

Monday, June 26, 2006

Viruses as engines of evolution

There are two recent reviews about viral origins and contributions to life on earth at the open-source journal Genome Biology here and at Nature here . As parasites, modern viruses have evolved strategies for incredible levels of compaction, but this means their very compressed genomes do not leave a lot of evidence of their origin. There has been a huge amount of progress on this problem as more and more viruses get sequenced and especially with the discovery of "giant viruses" such as mimivirus. Genomic methods are being used to discover viruses literally everywhere, many of which contain previously unknown genomic sequences.

With the new evidence and new ideas, it looks possible that viruses evolved from a very ancient, independent branch on the tree of life. But here's where the story gets pretty wild- perhaps viruses, sporting the first DNA in order to evade RNA defenses, actually made the very first nucleus. In this case there's a little bit of virus in all of us.

The Nature article highlights that viruses in the present-day world grab sequences from their hosts and each other. This mixing of genetic information itself can shuffle genes between viruses and even animals, meaning that genes are in effect pooled across an entire population:

"When you look at a group of viruses, such as the algal viruses, there seems to be a very, very small core of conserved genes," says Curtis Suttle, a microbiologist at the University of British Columbia in Vancouver, Canada. "The rest is almost like a super-organism — a massive pool of genetic information that's being shared among all these different viruses." (from the Nature review).

Wow, the borg is here!
UPDATE: And we are the borg- a nice writeup from a few weeks back by Dan Vergano at USA Today about how humans and the bacteria in their gut together make a superorganism. Something just made me think of Taco Bell.

Saturday, June 24, 2006

Science is the seed corn

Check out emptywheel at the next hurrah for an impassioned appeal that the U.S. continue to invest in the sciences, and that specifically a political effort is made to improve the near-term funding of the NIH. I believe that science and technology are the keys to future American prosperity.

Tuesday, June 20, 2006

Supply and demand- laser eye surgery and the military

There's a really interesting article in the NY Times about how the wide availability of corrective laser eye surgery in the Navy is affecting the application pools for the various postgraduate jobs. The big winners are aviation and special forces, both of which require perfect vision in applicants. They now select from a much larger pool. One loser is submarines, who used to get the glasses wearers but now have trouble filling their quota.

But don't worry- in 50 years, aerial combat will be perfomed drones, controlled by 10-year-olds on their PlayStation Xs-- and glasses will end up being cool.

Monday, June 19, 2006

The "HIV resistance mutation" might be very old

This month's Trends in Genetics has an update on the story of CCR5-delta32, a human mutation present at high frequency in Europeans and Western Asians but rare outside this region. People who are homozygous for this mutation are resistant to infection by HIV. It has been thought that the allele has been under positive selection, that is, that it has improved the survival of people carrying it, long before HIV was around, and might have therefore have conferred resistance to some other epidemic such as plague or smallpox.

The update talks about data that CCR5-delta32 might not have been historically under such strong selection as previously thought. The main new argument is that the mutation has been found in Bronze age bones, which means it has been around for a long time and might not be ramping upward in frequency over time as would be expected for a resistance gene. Secondly, an analysis called linkage disequilibrium, used to show evidence for positive selection, has been repeated with larger data sets and gives more ambiguous results.

Even though the population measurements for this mutation are less certain in humans, the evidence that CCR5 is critical for the timecourse of HIV infection is still very strong, and the mutation might still be an interesting marker for Northern European migrations (i.e. the Vikings).

An open-access discussion of CCR5-delta32 is here . I have blogged about this mutation and some interesting historical hypotheses here .

Sunday, June 18, 2006

Two-tongued but not tongue-tied

"I speak French to my ambassadors, English to my accountant, Italian to my mistress, Latin to my God, and German to my horse."
Frederick the Great

True multilingual people have exquisite control of what language comes out of their mouth at any given time. (In contrast, I always mishmash my languages, putting German prepositions into my French.) At a cognitive level, this skill involves hearing and understanding the language and formulating a reply, while suppressing the other languages, as if a "language switch" is at work. But in MRI images of these people during writing or speech, the brain activity patterns are very similar no matter what language is being used. This is probably because most of the brain concerned with meanings embedded in the language is going to be independent of the language used.

A recent paper in Science used a trick to try to locate brain regions which were directly related to the choice of language at the level of words and meanings. The authors had multilingual subjects read word pairs, in which the paired words either showed a close relationship (i.e. trout-salmon) or were not closely related (trout-horse). By varying whether the words within the pair were obtained from the same language or different languages, they sought to specifically trigger the brain region that coupled language to meaning. (There's no telling what havoc recent American english adoptees such as 'angst' or 'samizdat' would wreak in this test!)The testees were wired up to PET scans or functional MRI to measure their brain activity during these tests.

Now this test is still not so simple, because differences in shapes and lengths of the words (German words were on average 7% longer than the English equivalents) in different languages will affect brain areas without being specifically concerned with the meaning of the words. Despite these difficulties, the scientists were able to see two new effects with this test: an area in the left temporal lobe was activated differently depending on the relationship between the word pairs, without being affected by the language; and activation in an area called the left caudate was reduced in same-language pairs compared to different-language pairs.

The left caudate is a very interesting candidate location for an internal "language switch" because of earlier data from patients with damage near this brain area. These people can understand languages, but spontaneously switch between languages in their own speech and writing.

I would be interested to see the social aspects of this language switch, as hinted at by the quote from Frederick the Great. There must be some sort of recognition for what language is best for a given audience. My wife and have I frequently noticed that peoples' speaking styles differ depending on the language. A person can be fairly formal and courtly in French, for example, and quite casual in English. I am not in full command of any language besides English, but it seems that a sort of style or swing-- a cultural expectation-- attaches itself to languages.

UPDATE: My wife pointed out that languages don't just have words but also grammar- which I again muddle, speaking French with German word-order. A little discussion of word-order differences in Basque-Spanish bilinguals is available here although I couldn't find a publication.