Molecular Basis of Biological Clocks

   Recently, scientists have discovered that these ‘clocks’ are controlled by special genes that regulate the various rhythmic changes that occur in an organism during a day (circadian rhythms). This includes physiological changes throughout the body, including temperature, hormonal levels, and blood pressure. Even though it is not entirely clear how the resulting ‘clock’ proteins work, it is known that they are shared by nearly all organisms and that their main activity centers around the hypothalamus and the pineal gland in birds and mammals. Furthermore, light-sensing photoreceptors have been found to play a crucial role in keeping the biological clocks in tune.
The most direct application of research in this area is to alleviate potential health problems resulting from having a biological clock that is not in aligned with the environment. Problems such as jet lag, insomnia, and even depression can be alleviated simply by providing light at the correct times to successfully reset the biological clock.

What are the potential evolutionary advantages of having such a system. This eventually led me to the simplest organisms known to have an internal biological clock: the Blue-green algae (cyanobacteria). Even though many scientists initially believed that a daily biological clock would be irrelevant to organisms that divide in less than a day, cyanobacteria proved to be an exception. These organisms are of particular interest since they need to carry out two biochemically incompatible processes in order to survive: photosynthesis and nitrogen fixation. Cyanobacteria solved this incompatibility by carrying out photosynthesis during the day and nitrogen fixation at night (Shweiki, 2001). Thus, keeping track of temporal information has the potential of greatly optimizing the execution of these processes. This was corroborated by experiments that proved that bacteria whose biological clock is in tune with the naturally occurring light-dark cycle carried out this processes more efficiently, and in fact, where able to grow up to 30% faster than bacteria culture without tuned biological clocks (Salisbury, 2002).  The previous findings suggest that animals with a tuned biological clock do have an evolutionary advantage. Considering that nearly all organisms have this mechanism, it seems to be crucial for survival, regardless of whether it evolved in parallel or as a result of convergent evolution.

But how would humans benefit from having such mechanisms? Extrapolating from the previous findings, one can expect that early human civilizations that were in tune with the environment could have a strong advantage in carrying out more efficient hunting and gathering or even agriculture. Furthermore, Edery (2000) suggests that having a tuned biological clock is advantageous when adverse weather conditions caused humans to take shelter in caves for long periods of time.

The brain briefings article mentions the timing of light as the exclusive source of information to keep the biological clock in tune, which is not surprising considering that light is a reliable measure of daily cycles, in particular of day and night changes. However, I wanted to explore if there were any other sources of information that could be potentially used by this system. Closely related to the timing of light, I found that some species of birds and insects use the azimuth of the sun (the sun’s distance to the horizon) as a source of temporal information especially relevant when traveling long distances. Using the azimuth, which is closely related to the amount of light, seems to be an evolutionary adaptation for species that needed more information about the time of the day. But what other alternate sources of information are there that are not evidently related with the amount of light?

Initially, it seems that in some locations animals could derive temporal information based on temperature changes, since usually the night is colder than the day. This can be advantageous by ensuring that the maximum activity of an organism is carried out when the temperature is most appropriate (such as diurnal animals responding to colder temperatures and limiting their activity to the daytime). However, it was found that the period of the circadian rhythm does not vary with the frequent changes of temperature in the environment (Edery, 2000).

In the search of more complex sources of temporal information, Shweiki has suggested that cyclic patterns of gravity resulting from daily patterns of interactions between the earth and the moon gravitational fields could be used to know the current time. However, after several experiments, it is not clear if the daily variations in gravity can be sensed and processed by organisms (Shweiki, 2001). Hopefully, in the future scientists will be able to understand fully the level of sensitivity of the system to gravitational information, but also to more complex temporal cues such as atmospheric gas content and radiation levels.

In order to elicit the most appropriate behavioral responses whether it is on the shorter winter days or at the seemingly odd day times in a hotel room in Tokyo, biological clocks provide humans and other animals with vital information for survival. As research progresses, health problems associated with out of tuned biological clocks will be alleviated as humans adapt to a faster pace of life.