The ticking of life's clock

The life of all organisms is determined by an “interior clock,” which controls cell division, heartbeat, respiration, the amount of sleep required and many additional vital functions. Even single-cell organisms are guided by these chronobiological rhythms. “It has been known for a long time that there are various biological rhythms that all have period lengths around 24 hours,” explains Prof. Dr. Friedemann Schrenk of the Senckenberg Research Institute in Frankfurt, and he adds, “However, this ‘circadian rhythm’ does not explain the high diversity within the animal kingdom and the different pace of the various life cycles.”

Based on the question how mammals increased in size in the course of their evolutionary history, Schrenk, in cooperation with his colleague, Prof. Dr. Timothy G. Bromage, an honorary member of the Senckenberg Society for Nature Research and a professor at the New York University College of Dentistry, discovered a new, multi-day (‘multidian’) chronobiological rhythm. “If you were to scale the tissue of a mouse to the size of a human, this human would not be viable. The mouse tissue contains a large number of metabolic cells to enable the fast rhythm of life of these small mammals – a larger mammal would have to ingest enormous amounts of food to sustain its organism. This begs the question how small rodents were able to grow into huge animals in the course of geological history,” says Bromage in summary of this topic.

The two paleoanthropologists quickly understood that there had to be a mechanism that regulates the speed of cell division and thus enables the animals’ growth. Proof of their theory was found in the tooth enamel of domestic pigs – fine growth lines in the enamel that reveal daily rhythms. “In addition, there are the so-called striae of Retzius – parallel lines running through the tooth enamel – that occur at intervals of several days,” adds Bromage.In the context of the extensive “Paleobiomics” research program, which was financed by the Max Planck Research Prized awarded to Bromage, the German-American research duo compared the Retzius lines in the teeth of 33 pigs with blood samples taken from the animals. The blood samples, which were taken over a period of 14 days, contained metabolites – intermediate byproducts from metabolic processes – and ribosomal DNA, which occurred in a 5-day rhythm. “This was congruent with our results from the pigs’ teeth – there, the Retzius lines also occurred in 5-day intervals,“ rejoices Schrenk, and he continues, “The metabolites indicate that the rhythm is connected to a complex system of bodily functions – first in this context is the cell division, a basis for the growth of animals.”

The teeth of additional groups of mammals reveal that the length of this newly discovered rhythm varies, depending on the animals’ size. “For example, in small monkeys the intervals are shorter than in in the great apes,” adds Schrenk. The team of scientists hopes to be able to support this thesis in a further step through blood analyses of additional groups of animals.     

The discovery of the new rhythm could also be of significant benefit for us humans: “If we know the exact days when our cell production works at full blast, we could, for example, employ certain medications targeted toward specifically strengthening the organs ‘neglected’ at this particular point in time,” offers Bromage as a preview for the future.