I heard an argument that the biological clock provides an ability for organisms to predict fluxes in available nutrients and contaminants in the environment. By predicting the future reliably, an organism can gear up its metabolism in advance, or shut down unnecessary processes to conserve resources, so there is a clear selective advantage to organisms with a biological clock. Hence we see clocks in plants, fungi, animals and bacteria, and in some Archaea. I am not sure that the clocks are always strictly homologous though, it may be that the clock evolved multiple times, although this paper describes a conserved protein class, showing circadian rhythms in constant light after entrainment, across all domains of life: http://www.nature.com/nature/journal/v485/n7399/full/nature11088.html

They also show a nice brief review on evolution of clock mechanisms, and mention that the Kai oscillator is the most ancient clock mechanism, and is thought to have evolved around 2.5 billion years ago.

My guess for organisms without a clock would be to look for ones which have lost their clock (due to living in a place without daily environmental fluctuations), or which split from the rest of life more than 2.5billion years ago, when the peroxiredoxins are hypothesised to have originated (are there any such thing?). You might find such creatures in the deep places, underground or deep ocean vents, although I'm not sure whether there are even daily environmental fluctuations in these places due to gravity.

I guess you could find some genomes from these things and scan for absence of homologues of the Kai proteins, and measure the peroxiredoxins mentioned in the article above and show that they don't cycle, but then again absence of a detectable signal is not a great proof!

Hopefully someone else may know a model 'non-circadian' organism?

Dear Jonathan

Thanks a lot for your prompt, exhaustive and informative reply. This is exactly the kind of information I was looking for. Based on the research done on the topic and on your explanations, it is therefore very unlikely to imagine that a good number of species would not have a biological clock, and such potential organisms would indeed be primitive or live in places with little or no environmental fluctuations. Let's see if other colleagues might have knowledge of such 'aliens'.

Thanks again for your precious assistance. Kind regards

Emmanuel

Emmanuel, your question is too preemptive, it is built on incorrect assumption from the start. The term "clock" itself requires definition. It is often used in reference to various periodic processes on molecular level. More specific definition calls for a specific adaptation that stabilized the period of an oscillatory molecular process and a function that requires constant period. Preparing for impending shift in environment is one of such possible functions, but not the only one. Periodicity is not only dictated by environment or internal clock. It is also a property of any complex biological system compartmentalized in space. Orchestration of multiple rhythms is among the first functions of a living cell. In this sense there could not possibly be any clock-less organisms, it would be an oxymoron. There are plenty of organisms with internal rhythms detached from the diurnal variation in light and temperature. They may lack the circadian clock, but they are not entirely aperiodic or clockless.

Dear Andrey

Thank you so much for your reply and sorry for asking the wrong question. I am not specialised in this field at all, reason why I am happy that top-notched colleagues educate me in the matter. It's good to hear that some organisms do not necessarily have a circadian clock, but possess other internal rhythms. My questions for you would be the following ones:

1) Can you please name a few of those organisms with internal rhythms that differ from the circadian clock?

2) Do you know whether such organisms could possibly exhibit a near 24-hour ('daily') cycle due to synchronisation with environmental conditions (photoperiod, temperature, etc.)

Ultimately, I am trying to find out whether it is possible for an organism (and especially an animal) to exhibit a 'daily' cycle without possessing a circadian clock. In other terms, is it possible to find an animal that, put under constant dark or light conditions, would not show any circadian rhythm, but would exhibit a 'daily' rhythm under natural conditions.

Thanks in advance

Emmanuel

Emmanuel, don't search too far. Take yeasts, S.cerevisiae, for example. It fits.

Andrey, you might have missed this:

http://www.ncbi.nlm.nih.gov/pubmed/20133849