21.5 Epilogue
The statement put forth by Pasteur (1885) is worth reviewing in the context of
microbes and their hosts: “Life would not long remain possible in the absence of
microbes.” Gilbert and Neufeld (2014) addressed Pasteur’s statement in an exercise
to describe the evolutionary changes that would occur after eliminating microbes
from the rest of contributors to life on Earth but at the same time retaining mitochondria and chloroplasts, without which life would not exist. They then asked: “Or
would it?” The answer is that life might be feasible, but it would be very different
than it is now on Earth, but how different is difficult to say since so much of life on
Earth as we know it depends on the microbial partners that live in symbiosis with
their larger eukaryotic associates. In any case, it is highly unlikely that the diversity
seen in life on Earth described herein would ever have come about, much less been
maintained if any parameter crucial for hologenome survival had been different.
The processes whereby plants and their microbiomes adapt to their environments,
whether past, present, or future, depends on not only on microbes and their plant
hosts, but also on external factors such as geology, climate, water, temperature, and
serendipity to some extent, several of which are influenced by living organisms. To
rephrase L. Pasteur’s quote: “Life as we know it would not long remain possible in
the absence of microbes.” Microbes via their interactions with other organisms in an
environment that facilitated symbiotic interactions have given us an evolutionarily
diverse and for us, livable world. Recognizing how long it took for life to evolve,
diversify, and create Earth’s complex symbiotic environment may help us preserve
it—by alerting us to the fact that maintaining as much of the symbiotic world as we
can is not a trivial task, especially in our ever changing world.
Acknowledgements We thank the Fulbright Foundation for supporting S. Kouas’ stay in the
Hirsch lab, and also acknowledge the long-term grant support from the Shanbrom Foundation to
study arid soils and their microbes. Thanks to S.J. Kirchanski for his helpful comments on the
manuscript, and we also extend our gratitude to J.W. Schopf, J.F. Basinger, W.E. Stein, T. Algeo,
D. Murphy, and J. Shen-Miller for their helpful comments, suggestions of papers, and answers to
various questions sent by email. Allan Chong (UCLA) did the artwork for Fig. 21.2. We also thank
Christon J. Hurst for his support and helpful comments.
Finally, we appreciate P. G. Gensel and D. Edwards for writing the book “Plant Invade the Land.
Evolutionary and Environmental Perspectives”, which was an important reference for this chapter.
Saber Kouas
436
S. Kouas et al.
The statement put forth by Pasteur (1885) is worth reviewing in the context of
microbes and their hosts: “Life would not long remain possible in the absence of
microbes.” Gilbert and Neufeld (2014) addressed Pasteur’s statement in an exercise
to describe the evolutionary changes that would occur after eliminating microbes
from the rest of contributors to life on Earth but at the same time retaining mitochondria and chloroplasts, without which life would not exist. They then asked: “Or
would it?” The answer is that life might be feasible, but it would be very different
than it is now on Earth, but how different is difficult to say since so much of life on
Earth as we know it depends on the microbial partners that live in symbiosis with
their larger eukaryotic associates. In any case, it is highly unlikely that the diversity
seen in life on Earth described herein would ever have come about, much less been
maintained if any parameter crucial for hologenome survival had been different.
The processes whereby plants and their microbiomes adapt to their environments,
whether past, present, or future, depends on not only on microbes and their plant
hosts, but also on external factors such as geology, climate, water, temperature, and
serendipity to some extent, several of which are influenced by living organisms. To
rephrase L. Pasteur’s quote: “Life as we know it would not long remain possible in
the absence of microbes.” Microbes via their interactions with other organisms in an
environment that facilitated symbiotic interactions have given us an evolutionarily
diverse and for us, livable world. Recognizing how long it took for life to evolve,
diversify, and create Earth’s complex symbiotic environment may help us preserve
it—by alerting us to the fact that maintaining as much of the symbiotic world as we
can is not a trivial task, especially in our ever changing world.
Acknowledgements We thank the Fulbright Foundation for supporting S. Kouas’ stay in the
Hirsch lab, and also acknowledge the long-term grant support from the Shanbrom Foundation to
study arid soils and their microbes. Thanks to S.J. Kirchanski for his helpful comments on the
manuscript, and we also extend our gratitude to J.W. Schopf, J.F. Basinger, W.E. Stein, T. Algeo,
D. Murphy, and J. Shen-Miller for their helpful comments, suggestions of papers, and answers to
various questions sent by email. Allan Chong (UCLA) did the artwork for Fig. 21.2. We also thank
Christon J. Hurst for his support and helpful comments.
Finally, we appreciate P. G. Gensel and D. Edwards for writing the book “Plant Invade the Land.
Evolutionary and Environmental Perspectives”, which was an important reference for this chapter.
Saber Kouas
436
S. Kouas et al.