event), and gave rise to the mitochondrion (Imachi et al. 2020). Then between
540 and 520 million years ago there was the extraordinary Cambrian evolutionary
explosion that resulted in the appearance of most of the existing animal phyla. The
earliest vertebrates appeared 20–30 million years after the Cambrian explosion. Most
experts think the crucial endosymbiotic event that created the mitochondrion
occurred only once, so ultimately humans, like all eukaryotic life forms, evolved
from a blend of 2 or more microbes. Recent biotechnology has revealed that about
65% of human genes originated in Bacteria, Archaea, and eukaryotic microbes
(Domazet-Loso and Tautz 2008). This is strikingly true of the genes enabling
synthesis of the neurotransmitters that are crucial to the brains of which we are so
proud (Iyer et al. 2004). So we evolved from Bacteria, Archaea, and eukaryotic
microbes, and we took most of our genes from them. But we also carry a vast
community of them within our bodies.
Our guts contain symbiotic organisms (the microbiota) that are at least as
numerous as the human cells in our bodies, and 30% or more of the small molecules
in our peripheral blood, many of which have profound effects on our physiology, are
products of the metabolism of these microbes (Wikoff et al. 2009). How did this
situation evolve? Early in evolution, the organisms that inevitably found their way
into the gut were separated from the host by a chitin barrier (Nakashima et al. 2018),
a structure that persists in arthropods and annelids. In chordate invertebrates, such as
tunicates, the chitin mesh is embedded in a mucin gel, and the gut bacteria are still
rigorously separated from the gut epithelium. In the most primitive vertebrates (the
ray-finned fish) a more substantial mucus layer is secreted by intestinal goblet cells,
and this mucus covers the epithelium. However, the mucus layer is still separated
from the lumen by a chitin membrane. Finally, in mammals the chitin layer is lost
entirely and complex mucus layers interact with, and nourish organisms, many of
which adhere to the mucus and modulate the function of the underlying cells
(Nakashima et al. 2018). It is interesting that this parallels the situation in plants
where organisms are attracted and nourished by molecules secreted from the roots,
and then take part in symbiotic two-way signalling and exchange of nutrients.
18.1.2 Evolution of the Adaptive Immune System
This complex mucin barrier, devoid of the chitin mesh, allowed a much more
intimate exchange of signals and metabolites between host and microbiota, and a
much more complex community of organisms, with far greater numbers and diversity. Vertebrates co-evolved with this microbiota, which took on roles in the
development and function of essentially all organs, including the brain. For example,
studies of mice have revealed that germ-free animals delivered into a sterile environment by Caesarean section have abnormal brains and abnormal reactions to
stress. These abnormalities seemingly can only be corrected by re-installing a normal
microbiota in the early weeks of life (Sudo et al. 2004; Diaz Heijtz et al. 2011).
Managing, tolerating, and “farming” this physiologically essential microbiota, while
18 Darwinian Medicine: We Evolved to Require Continuing Contact with the. . .
329
540 and 520 million years ago there was the extraordinary Cambrian evolutionary
explosion that resulted in the appearance of most of the existing animal phyla. The
earliest vertebrates appeared 20–30 million years after the Cambrian explosion. Most
experts think the crucial endosymbiotic event that created the mitochondrion
occurred only once, so ultimately humans, like all eukaryotic life forms, evolved
from a blend of 2 or more microbes. Recent biotechnology has revealed that about
65% of human genes originated in Bacteria, Archaea, and eukaryotic microbes
(Domazet-Loso and Tautz 2008). This is strikingly true of the genes enabling
synthesis of the neurotransmitters that are crucial to the brains of which we are so
proud (Iyer et al. 2004). So we evolved from Bacteria, Archaea, and eukaryotic
microbes, and we took most of our genes from them. But we also carry a vast
community of them within our bodies.
Our guts contain symbiotic organisms (the microbiota) that are at least as
numerous as the human cells in our bodies, and 30% or more of the small molecules
in our peripheral blood, many of which have profound effects on our physiology, are
products of the metabolism of these microbes (Wikoff et al. 2009). How did this
situation evolve? Early in evolution, the organisms that inevitably found their way
into the gut were separated from the host by a chitin barrier (Nakashima et al. 2018),
a structure that persists in arthropods and annelids. In chordate invertebrates, such as
tunicates, the chitin mesh is embedded in a mucin gel, and the gut bacteria are still
rigorously separated from the gut epithelium. In the most primitive vertebrates (the
ray-finned fish) a more substantial mucus layer is secreted by intestinal goblet cells,
and this mucus covers the epithelium. However, the mucus layer is still separated
from the lumen by a chitin membrane. Finally, in mammals the chitin layer is lost
entirely and complex mucus layers interact with, and nourish organisms, many of
which adhere to the mucus and modulate the function of the underlying cells
(Nakashima et al. 2018). It is interesting that this parallels the situation in plants
where organisms are attracted and nourished by molecules secreted from the roots,
and then take part in symbiotic two-way signalling and exchange of nutrients.
18.1.2 Evolution of the Adaptive Immune System
This complex mucin barrier, devoid of the chitin mesh, allowed a much more
intimate exchange of signals and metabolites between host and microbiota, and a
much more complex community of organisms, with far greater numbers and diversity. Vertebrates co-evolved with this microbiota, which took on roles in the
development and function of essentially all organs, including the brain. For example,
studies of mice have revealed that germ-free animals delivered into a sterile environment by Caesarean section have abnormal brains and abnormal reactions to
stress. These abnormalities seemingly can only be corrected by re-installing a normal
microbiota in the early weeks of life (Sudo et al. 2004; Diaz Heijtz et al. 2011).
Managing, tolerating, and “farming” this physiologically essential microbiota, while
18 Darwinian Medicine: We Evolved to Require Continuing Contact with the. . .
329