alter the competitive balance by not expending unnecessary energy to maintain
metabolic activities that have become unuseful.
Chloroplasts, considered as inherited photosynthetic organelles, would have
begun as undigested cyanobacteria, and since that event of indigestion the
cyanobacteria have followed an evolutionary pathway similar to that of the mitochondria. The combination of chloroplasts plus mitochondria has meant that aerobic
respiration associated with mitochondria may help to carry the hosts through periods
when there is no sunlight available for photosynthesis. It is a combination which has
enabled plants to reach a tremendous level of aquatic and terrestrial dominance.
Although we often think of chloroplasts as being a phenomenon associated with
algae and vascular plants, there additionally are animal groups which interestingly
have utilized ingested endosymbiotic algae, and sometimes adopted only chloroplasts that have been scavaged from ingested algae, as a photosynthetic source of
energy and fixed carbon.
Howe et al. (2008) addressed the concepts of primary photosynthetic plastids
having initially arisen by non-photosynthetic eukaryotic hosts engulfing photosynthetic prokaryotes, and those photosynthetic eukaryotes in turn having been ingested
by non-photosynthetic eukaryote hosts to form secondary plastids.
Martin et al. (2015) have presented their perspective, and a set of suggestions,
regarding the processes which may have led to evolution of a cell nucleus, mitochondria and chloroplasts, and successive gene transfer from intracellular organelles
to the host cell genome.
26.1.2 A Fixation with Nitrogen
All of these concepts represent ways of cheating, after which other groups must try to
at least level the competition or hopefully best the competition by trying something
new. I will address these pathways to mitochondria and to chloroplasts later in this
essay. I also will consider the question of what might have come next after the
competitors had evolved to the point that all plants had both mitochondria and
chloroplasts. Perhaps the next idea was finding a microbial symbiosis with organisms that had nitrogenase enzymes and therefore could provide a source of fixed
nitrogen. The oldest of these symbioses could have been an association with the
cyanobacteria whose descendants we know as Anabaena. There have been competing symbioses with the proteobacteria whose present descendants include Rhizobium, and the actinobacteria whose present descendants include Frankia.
26.1.3 Some Other Symbioses that Have Become Vital
During the late 1800s, it was possible to fully address the subject of microbial
symbiosis in a single lecture. Today, not even a very large book could suffice to
26 The Game of Evolution Is Won by Competitive Cheating
547
metabolic activities that have become unuseful.
Chloroplasts, considered as inherited photosynthetic organelles, would have
begun as undigested cyanobacteria, and since that event of indigestion the
cyanobacteria have followed an evolutionary pathway similar to that of the mitochondria. The combination of chloroplasts plus mitochondria has meant that aerobic
respiration associated with mitochondria may help to carry the hosts through periods
when there is no sunlight available for photosynthesis. It is a combination which has
enabled plants to reach a tremendous level of aquatic and terrestrial dominance.
Although we often think of chloroplasts as being a phenomenon associated with
algae and vascular plants, there additionally are animal groups which interestingly
have utilized ingested endosymbiotic algae, and sometimes adopted only chloroplasts that have been scavaged from ingested algae, as a photosynthetic source of
energy and fixed carbon.
Howe et al. (2008) addressed the concepts of primary photosynthetic plastids
having initially arisen by non-photosynthetic eukaryotic hosts engulfing photosynthetic prokaryotes, and those photosynthetic eukaryotes in turn having been ingested
by non-photosynthetic eukaryote hosts to form secondary plastids.
Martin et al. (2015) have presented their perspective, and a set of suggestions,
regarding the processes which may have led to evolution of a cell nucleus, mitochondria and chloroplasts, and successive gene transfer from intracellular organelles
to the host cell genome.
26.1.2 A Fixation with Nitrogen
All of these concepts represent ways of cheating, after which other groups must try to
at least level the competition or hopefully best the competition by trying something
new. I will address these pathways to mitochondria and to chloroplasts later in this
essay. I also will consider the question of what might have come next after the
competitors had evolved to the point that all plants had both mitochondria and
chloroplasts. Perhaps the next idea was finding a microbial symbiosis with organisms that had nitrogenase enzymes and therefore could provide a source of fixed
nitrogen. The oldest of these symbioses could have been an association with the
cyanobacteria whose descendants we know as Anabaena. There have been competing symbioses with the proteobacteria whose present descendants include Rhizobium, and the actinobacteria whose present descendants include Frankia.
26.1.3 Some Other Symbioses that Have Become Vital
During the late 1800s, it was possible to fully address the subject of microbial
symbiosis in a single lecture. Today, not even a very large book could suffice to
26 The Game of Evolution Is Won by Competitive Cheating
547