Multicellular life

All multicellular life starts as a single cell. Copies of the DNA in that cell must eventually occupy almost every one of the trillions of cells in a human body. For that to happen, the DNA in the original cell must replicate itself many times. The key to this replication is the famous double helix. When two strands of DNA— let s call them X and Y—separate, each strand can assemble the other. X builds a new Y, forming a fresh double helix. Y does the same thing. This doubles the number of DNA molecules. This mechanism depends on the two strands of DNA being able to hold together under normal conditions, yet unwind easily. That is where hydrogen bonds come in. 

As multicellular life forms reached a sufficient degree of complexity, they improved their physical separation from the water medium, allowing migration to land and continued evolution in novel environments. In terrestrial plants and animals, water is no longer both external and internal, but solely the internal milieu, the milieu interieur, in constant flux with the environment yet carefully separated from it Such, in a nutshell, is the biophysical role of water as a medium. 

Stanley SM (1973) An ecological theory for the sudden origin of multicellular life in the late Precambrian. Proc Nat Acad Sri USA 70 1486-1489 Stefels J (2000) Physiological aspects of the production and conversion of DMSP in marine algae and higher plants. J Sea Res 43 183-197... 

Many reservoirs of biologically important elements turn over so rapidly at the Earth surface that modest differences between input and output could either remove them completely or double their concentrations in the geologically short span of thousands to millions of years. Despite this precarious situation, evidence indicates that the Earth has been remarkably stable in its habitability, allowing the chain of multicellular life to persist for more than a half biUion years. Such continuity requires a conspiracy of feedbacks within the Earth system that stabilizes the availability of life s key ingredients. Yet we know little about these feedbacks, how they develop, or the constraints that they impose on the environment and life. Beyond these... 

Life would go on, but the creatures would resemble us even less than we do the dinosaurs, and less than the dinosaurs resemble the single cells from whence they came. But those single cells were responsible for producing the oxygen in the atmosphere that eventually accumulated to the point that allowed multicellular life, so it might be that this cycle would eventually repeat. We are used to thinking linearly, in terms of a beginning and an end, because that is how we reason. But nature is not so constrained. Nature tends to appreciate cycles, and life has its ups and downs. 

Earth is a planet with an atmosphere, abundant liquid water, and an extensive biota that appears to be ubiquitous. The fossil record shows the presence of multicellular life forms from the late Precambrian to the present, and there is little doubt that microscopic forms of life have been present on Earth for several billion years. It may well be impossible to study chemical... 

All copper proteins which are known to be phylogenetically related to noncopper proteins - i. e., the small blue proteins, metallothioneins and some type 2 copper proteins - are found in single-celled organisms. Hemocyanins, blue oxidases, and those type 2 copper proteins whose functions are essential for multicellular life-forms are not phylogenetically related to non-copper proteins and do not occur in single-celled organisms. 

The Precambrian is drawing to an end. We have travelled down 3 billion years. There has been little to see but much has changed. Without these changes, the explosion of multicellular life that is soon to follow would have been impossible. I have argued that the changes were linked with rises in atmospheric oxygen. 

Comment. One could speculate that the unicellular and the early multicellular life forms existed in a highly stressed life style, and replicated rapidly without rest, or senescence. In contrast, in the mature multicellular hosts, the overwhelming number of the somatic cell populations encode just one particular task, divide seldom if ever, and keep the rest of their genomes silenced (under the control of silenced promoters). Stem cells are in the minority, divide asymmetrically, but seldom, otherwise they rest Haploid germ cells exert little activity until after become zygotes (the united sperm and the ovum). Thereafter, they dictate the ontogenesis of the individual. How their intron counts reflect to their life style ... 

Michoud RE. Evolution of individuality during the transition from unicellular to multicellular life. Proc Natl Acad Sci USA. 2007 104 8613-8. 

Another clue to the Cambrian explosion is communication. The neurotransmitters in the human brain are one of the most advanced forms of chemical communication, but this communication molecule is surprisingly old. Algae aren t even true multicellular life forms, yet they contain neurotransmitters. 

In fact, it is also a very long way from the first one-celled organism to the first multicelled organism. Biologists consider the appearance of the first multicellular life to be as big a step in the history of hfe on Earth as was the appearance of the first unicellular life. 

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