Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Sunday, January 30, 2011

Tick tock

As those of us in the northern hemisphere of this small rocky planet contend with the winter nights and days it can feel like our internal clocks get a little out of whack. However, we and many other organisms actually have an extraordinarily robust built in timing mechanism that carries us through a roughly 24 hour cycle. Birds do it, bees do it, even educated C. Elegans do it. The circadian rhythm is something that may be a global property of terrestrial life. Regardless of sunlight then living things tend to operate on a daily routine, from rest to activity, and from high to low metabolic activity.

The exact biochemical origins of this internal clock have been somewhat elusive. In last week's Nature two new works by O'Neill et al. shed some more light on the subject. A possibility has been that a transcription/translation feedback loop governing expression of certain 'clock' genes played a role in setting the 24 hour timer in organisms. O'Neill and colleagues seem to have found good evidence that there are additional, possibly superior, 'time-keeping' processes at play. In essence these are chemical 'oscillators' that behave like a well-tuned pendulum. Intriguingly this type of mechanism was already known to operate in the ancient cyano-bacteria. In tandem then perhaps both the purely chemical and gene mechanisms act like a self-correcting clock, keeping life to a consistent 24 hour timetable. The genetic coding for the chemical clock seems likely to be shared amongst organisms like ourselves and ancient bacteria.

This is all very interesting. However, it also raises a number of questions that I've not seen discussed in detail in these or related experiments. 24 hours is the rotation period of the modern Earth. The Earth-Moon system has been in constant dynamical evolution since the formation of the Moon about 4.53 billion years ago following a massive proto-planet collision. At present the gravitational tides due to the Moon are dissipating energy at a rate of a few Terawatts and slowing the Earth's rotation by about a couple of milliseconds a century. Other variations, like changing ice-caps, solar tides, even tectonic shifts tend to obscure this slowdown on short timescales but over millions of years there is little doubt that the Earth's spin has been slowing. At the same time angular momentum conservation means that the Moon is receding from us at a few centimeters a year - a fact confirmed by laser ranging.

The upshot is that it's quite possible that 4 billion years ago the Earth's daylength was only 12 hours. Geological evidence is scarce to non-existent that far back, but studies of material deposited on what were once tidal shorelines indicate that around 600 million years ago the day length was certainly more like 22 hours, and the slowdown should have been more extreme in the further past. So the intriguing question to ask is how the biochemical clocks, be they the genetic or chemical variety, adjust over the millenia to that shift? Or, to be provocative, is there some way we could use our understanding of the evolution of these mechanisms to independently test the physical changes to Earth rotation over hundreds of millions to billions of years?

Celestial mechanics probed by paleogenetics? That sure sounds like fun.


Eniac said...

Fun idea, yes, but not likely to succeed. The minute tweaks that are needed to change the period of the clock are likely to be completely buried in all the random and not so random genetic changes that otherwise go on over megayears, not to speak of gigayears.

Also, it may be worth mentioning that the biological clock is quite imprecise. Experiments in humans have shown that the clock has a period of roughly 25 hours when all external clues are removed. Light plays an important role in synchronization, but I would bet an alarm clock will also do it.

Caleb Scharf said...

Right. I think what *might* be possible though is to investigate just how 'fast' the current biological clock(s) can run - i.e. could the same mechanism operate for a 12 hour cycle? My guess would be that the same basic mechanism works, but since - as you say - today's human clock is 25 hours then that suggests a degree of inflexibility if that discrepancy is due to simply not being able to get to higher precision. Perhaps we need to subject some cyanobacteria to prolonged artificial day/night cycles - something someone may have done already?

Eniac said...

Thinking about it more, I think there might be some merit in this. In principle, we can infer entire ancient genomes by looking at enough current ones. If we can then express them and measure the cicadian rythm, we'll have a historical data point on length of day.

For example: We know all the genomic differences between chimpanzee and human (only 1% of nucleotides are different), and we also know which alleles are ancestral, from looking at other, less closely related genomes. If we simply construct a genome in which all alleles that are different between chimp and human are chosen to be ancestral, we have reconstructed the genome of the common ancestor of human and chimp. We can now synthesize this genome, transfer it into a chimp egg, implant in a chimp, raise the cub, and put it in an evenly lighted room to observe its rhythm. Then we would know what the length of day was at the time this ancestral species lived, approximately.

It gets more difficult going further back, but in principle the same should be possible with cyanobacteria, and billions of years. In some respects it would be easier, because there are less complications in expressing synthetic genomes in bacteria.

We would need to gather the sequences of very many existing species, all descending from a common, cyanobacterial ancestor back at the time that we are interested in. Once we reconstruct the ancestral genome (provided it is possible that far back), we pull a Venter and, voila!, our ancient cyanobacterium, ready to be tested for the length of its rhythm (if it had one...).

Speaking of Venter, he would probably be interested in doing this...

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