Living organisms consist of cooperating molecules. Thus, a reasonable way to envision the origin of Life is to postulate initial associations of cooperating molecules. In turn, teams of molecules that have figured out important functions with regard to metabolism, growth and replication, can be expected to enter into larger associations benefiting all participants. When such a larger association acquires a protective wall, we have the first primitive prokaryotic cell, that is, an organism with distributed genetic material. At the next level, we might expect two or more of such primitive organisms to combine their skills, to make more advanced bacteria or Archea. Further combination then yields a eukaryotic cell, after reorganizing genetic material into a nucleus. Finally, cells collaborate to make multicellular algae, plants and animals, which in turn enter into various types of symbiotic associations with other life forms. At the highest level of complexity, ecologic systems evolve which employ a mixture of organisms of all domains. From there to the "Gaia" concept is but a small step, where the entire living web of organisms is seen as one ecologic system "in charge" of keeping the planet habitable.
Symbiosis means "living together"- two or more organisms who share some part of their life processes. There are three types of symbiosis. In commensalism, one organism benefits from the relationship while the other derives neither benefit nor harm. When both profit from the symbiotic arrangement it is called mutualism. In parasitism, one organism benefits at the expense and even death of the other.
In 1932, the Russian botanist B. Keller opined that "in the cell nucleus we have a system consisting of the residues of primitive living units which have been altered in an extraordinary way and are highly specialized." He proposed that "at some distant time in its history the cellular nucleus passed through a stage when it existed as a colony of elementary living units similar to the colony stage through which multicellular organisms passed. Bacteriophages and genes are the remnants of those living units." [cit. in Oparin] Keller further noted that "chlorophyll grains also must have been at one time independent living units, simpler than the cell itself, but containing the green substance, chlorophyll." He assumes that the chlorobacteria are similar to those early chlorophyll-bearers. The most remarkably forceful statement in this regard is as follows: "This symbiosis of organisms, which was at first accidental, gradually became elaborated into a most intimate and permanent system in which the previously independent organisms acquired the character of organs of a single whole, the cell."
Serial Endosymbiosis Theory
Let's look again at our sketch of the components of a eukaryotic cell.
A section through a eucaryotic cell, with its prominent nucleus, in which the long DNA
molecules (carrier of genetic instructions) lie tangled up. Other prominent cell components or organelles
are floating in the endoplasmic fluid between the nucleus and the outer cell wall
. These organelles include the mitochondria (responsible for energy budget), lysosomes (digestion), the reticulum and the Golgi
apparatus. (Courtesy: Vaike Haas at: http://www.bact.wisc.edu/bact303/Vaikescells
Where did all these cell structures (organelles) come from? We briefly reviewed their functions in the last section, but not how they came to be inside the cell walls. The most obvious component is the nucleus, containing the genetic code in the form of DNA. The presence of the nucleus distinguishes our eukaryotic (true nucleus) domain from the domains of the prokaryotic (pre-nucleus) bacteria and archaea which have their DNA in the form of small circular "plasmids".
The American biologist Lynn Margulis (born 1938) (now at the University of Massachusetts, Amherst) is currently the leading proponent of the importance of symbiosis in the development of complex life. In her book "Origin of Eukaryotic Cells" (1970), she argued that eukaryotes derive from colonies of prokaryotes. Organelles within cells, such as the mitochondria, in her view, were once free-living primitive prokaryotes. They have, "over a long period of time, established a hereditary symbiosis with ancestral hosts that ultimately evolved into animal cells." [cit. in Dictionary of Scientists, 1999] It is not known, of course, precisely how such symbiosis arose. It may have started as a hostile takeover that developed into a cooperative partnership. It may have started as a modestly beneficial association, with different organisms seeking each others company because one produced as waste what the other needed as resource. In any case, apparently it is a common event that occurred again and again.
The concept that new organelles and functions are incorporated into cells as evolution proceeds, through symbiosis, is referred to as "serial endosymbiosis theory - SET". Margulis' short book "Symbiotic Planet" discusses the discovery of the role of symbiosis in the evolution of life on Earth and the creation of new species by symbiogenesis at a popular and very readable level.
The basics of making a eukaryotic cell according to SET are as follows. The first step was the merger of a heat and acid-loving archaeon with a swimming bacterium like a spirochete to form the nucleocytoplasm, found in all fungal, plant and animal cells. The conversion of free-swimming spirochetes to cell-propelling undulipodia, cilia and so on is one of the more controversial aspects of SET. The other steps seem to have been confirmed by study of cell component genomes. The combined cell was the first swimming organism with a nucleus, a protist; neither plant nor animal nor fungus. It lived without oxygen, and was in fact poisoned by it. The nucleus arose not from engulfing another organism, but from the symbiotic merger of the first two organisms. There are no free-living bacteria that look much like bare nuclei.
The next step the combined organism had to achieve was the development of mitosis, the division of cell components during replication. The next symbiotic acquisition was the capture of an oxygen-breathing bacterium. Next came the ability to surround food material and create an interior space, a vacuole, where the food might be consumed at leisure. This was obviously a big improvement over the previous technique of nuzzling up to food, secreting digestive fluids and re-absorbing them. The first organism using internal digestion first appeared around 2 Ga, that is, 2,000 million years ago.
The final component was incorporated, presumably, when food became a partner. The now three-part cell efficiently engulfed and digested green photosynthetic bacteria. Some green bacteria lingered on and photosynthesized within their host, before being digested. Some hosts learned to hold back the digestive process, to give the bacteria more time to do their thing. A partnership was born, and eventually the green bacteria became chloroplasts. The merged cell now was a green micro-alga. Eventually it became the ancestor to the familiar macro-algae and plants.
Several other classes of membrane-bounded organelles occurring in the cytoplasm between the nucleus wall and the outer cell wall resemble bacteria in their behavior and metabolism. Examples are plastids, mitochrondria, and microtubules. Further study will reveal the ancestry of these highly modified colonies of symbionts. We are their descendants.
From the foregoing it is already clear that the concept of "species" is of limited use in elucidating the origin of life and its evolution during the early stages of history. The term species refers to interbreeding populations who produce fertile offspring, or to potentially interbreeding populations (if they were at the same place at the same time). It is a concept well suited for the classification of domesticated plants and animals and all the other life forms which have similar life histories. It is good to remember that the species concept originated in the 18th century, when species were still considered immutable and bacteria were unknown.
When it became clear that species are not immutable, the concept became fuzzy. Genetic changes accumulate over time. Eventually, once-related populations have evolved into separate and distinct species. But to draw the line where one species ends and another begins in such a series is an arbitrary exercise. Fortunately, evolution proceeds sometimes very slowly and sometimes quite rapidly, based on the changing morphology seen in fossil lineages, which makes it easier to draw a line. (This circumstance has been celebrated by the impressive label "punctuated equilibrium".)