Chapter 114 19.3 Connected branches on the tree of life

It was once thought that symbiosis (two organisms becoming one) could only happen to exotic, relatively isolated organisms like lichens.Since Lynn Margulies put forward the hypothesis that "bacterial symbiosis is the core event in the formation of progenitor cells", biologists suddenly discovered that symbiosis abounds in the microbial world.Since microbial life is (and has been) a major part of all life forms on Earth and is the primary workhorse of the Gaia hypothesis, widespread microbial symbiosis makes symbiosis a fundamental behavior both in the past and in the present. Contrary to the traditional picture of a population turbulent with small, random, incremental changes in its daily behavior before accidentally forming some new stable structure, Margulies wants us to consider An accidental phenomenon in which two normally functioning simple systems merge into a larger, more complex system.For example, a proven system for transporting oxygen inherited in one cell line may be tightly integrated with an existing system for gas exchange in another cell line.The symbiotic association of the two parties may form a respiratory system, and this development process may not be progressive.

Margulies suggests her own work on the symbiotic nature of nucleated cells as an example of biological history.These emerging cells, without having to go through billions of years of trial and error to reinvent a process, cleverly integrated photosynthesis and respiration performed separately by several kinds of bacteria.Instead, these membrane-forming cells integrate the bacteria and their informational assets into themselves, taking full possession of the daughter bodies to work for the cell mother.They appropriated the bacterial invention for themselves. In some cases, genetic strains (segments of bases) of symbiotic partners fuse together.A mechanism has been proposed for the cooperation of information required for this symbiotic relationship, known as gene transfer between cells.This transfer occurs with high frequency among bacteria in the wild.A system's proprietary information can be shuttled back and forth between different species.The new bacteriology holds that all the bacteria in the world are a single, genetically interacting superorganism, which absorbs and spreads genetic innovations among its members at breakneck speed.In addition, gene transfer between species also occurs (at an unknown rate) between more complex species, including humans.Each type of species is constantly exchanging genes, often with naked viruses acting as messengers.Viruses themselves are sometimes included in the symbiosis.Many biologists believe that there are large chunks of human DNA strands that are inserted viruses.And some biologists even think it's a cycle—the viruses that cause many of our diseases are surly bits of escaping human DNA.

If this is true, the symbiotic nature of cells has many lessons to teach us.First, it provides us with an example of a major evolutionary change that reduces the direct benefit to an individual organism (because the individual disappears), contrary to Darwinian dogma.Second, it also provides an example: Evolutionary change is not the accumulation of small and gradual differences, which is also contrary to Darwinian dogma. Large-scale routine symbiosis can give rise to many complex phenomena in nature—phenomena that seem to require multiple innovations to occur simultaneously.It would also provide additional conveniences for evolution; for example, symbiotic behaviors could only exploit the forces of cooperation rather than competition.At the very least, cooperation can foster a unique set of niches and a diversity that competition cannot provide, such as lichen systems.In other words, it unleashes another dimension of evolutionary space by augmenting the pool of biological forms.More than that, a little bit of symbiotic coordination at the right time can replace long periods of small changes.Evolutionary processes in reciprocal relationships can skip millions of years of individual trial and error.

Perhaps, without symbiosis, the evolutionary process can directly obtain nucleated cells, but it may take another billion or five billion years to complete this goal.Symbiosis recombines the various experiences and gains scattered across the spectrum of life.The tree of life is constantly beginning to shed its leaves and spread its branches in all directions.At the same time, symbiosis brings the forked branches of the tree of life back together so that they intersect.And evolution that incorporates symbiosis is more like a bush than a tree—the bush of life.If this picture is roughly correct, perhaps we should rethink our past and future.