be renewed as required. Therefore, it is not a surprise that allowing multiple sclerosis
(MS) patients to become infected with helminths they would have encountered in
childhood can stop progression of the disease (Correale and Farez 2011), whereas
trials of helminth therapy for MS in locations where helminths have not been
endemic for several generations are failing (Fleming et al. 2019; Charabati et al.
2020). Evolution turns the occasional into an option (via epigenetics), not into a
germ line-encoded necessity.
So we are neither in a state of evolved dependence on the common infections of
childhood, nor on helminth infections. Similarly, it is not possible that we are in a
state of evolved dependence on the microbiota of the modern home. Until relatively
recently our homes were derived from the natural environment, and built with
natural timber, thatch, and mud- or dung-based plaster. In sharp contrast, the modern
home is constructed from concrete, plastic, and biocide-treated timber and plasterboard (Sahlberg et al. 2013). The quite different microbiota of such a home cannot be
part of our evolutionary heritage so it is not surprising that detailed studies do not
find that modifications of the microbiota of modern homes by domestic cleaning
practices are linked to immunoregulatory disorders (Weber et al. 2015). As will be
discussed in detail later in Sect. 18.3, the relevant aspect of the microbiome of the
home is the extent to which it resembles that of the natural environment. . . . or of the
shelters and homes of our ancestors (Kirjavainen et al. 2019). Confusion about the
role of domestic cleaning has been exacerbated by the fact that the cleaning agents
themselves damage the lungs of children, but that is a separate issue unrelated to the
microbiome (Parks et al. 2020). These errors in our thinking have delayed progress.
Clearly, we need to ask ourselves which microbes really were inevitably present
during an evolutionarily significant part of our past. Then amongst these organisms,
we may find some on which we are in a state of evolved dependence. Finally, we will
need to explain the mechanism of this dependence so that we can provide the
relevant organism or mimic its function with novel medicines.
18.2 Microbiota from Humans and the Natural
Environment
We receive the necessary microbial inputs from other humans (especially our
mothers), and from the natural environment (Fig. 18.1). The fact that the gut
microbiota is a crucial part of our physiology, and that its composition is a major
determinant of health is now well-known and has been subject to numerous recent
reviews (Lynch and Pedersen 2016; Davenport et al. 2017). Moreover, we now
understand many of the lifestyle changes that are limiting or corrupting the transmission of the microbiota from family to child. These factors include caesarean
deliveries, lack of breastfeeding, inappropriate hygiene that limits mother–child
transmission, poor diet, and antibiotics during pregnancy and infancy. This last
point is particularly worrying. The relative risk of obesity is linearly related to the
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G. A. W. Rook
(MS) patients to become infected with helminths they would have encountered in
childhood can stop progression of the disease (Correale and Farez 2011), whereas
trials of helminth therapy for MS in locations where helminths have not been
endemic for several generations are failing (Fleming et al. 2019; Charabati et al.
2020). Evolution turns the occasional into an option (via epigenetics), not into a
germ line-encoded necessity.
So we are neither in a state of evolved dependence on the common infections of
childhood, nor on helminth infections. Similarly, it is not possible that we are in a
state of evolved dependence on the microbiota of the modern home. Until relatively
recently our homes were derived from the natural environment, and built with
natural timber, thatch, and mud- or dung-based plaster. In sharp contrast, the modern
home is constructed from concrete, plastic, and biocide-treated timber and plasterboard (Sahlberg et al. 2013). The quite different microbiota of such a home cannot be
part of our evolutionary heritage so it is not surprising that detailed studies do not
find that modifications of the microbiota of modern homes by domestic cleaning
practices are linked to immunoregulatory disorders (Weber et al. 2015). As will be
discussed in detail later in Sect. 18.3, the relevant aspect of the microbiome of the
home is the extent to which it resembles that of the natural environment. . . . or of the
shelters and homes of our ancestors (Kirjavainen et al. 2019). Confusion about the
role of domestic cleaning has been exacerbated by the fact that the cleaning agents
themselves damage the lungs of children, but that is a separate issue unrelated to the
microbiome (Parks et al. 2020). These errors in our thinking have delayed progress.
Clearly, we need to ask ourselves which microbes really were inevitably present
during an evolutionarily significant part of our past. Then amongst these organisms,
we may find some on which we are in a state of evolved dependence. Finally, we will
need to explain the mechanism of this dependence so that we can provide the
relevant organism or mimic its function with novel medicines.
18.2 Microbiota from Humans and the Natural
Environment
We receive the necessary microbial inputs from other humans (especially our
mothers), and from the natural environment (Fig. 18.1). The fact that the gut
microbiota is a crucial part of our physiology, and that its composition is a major
determinant of health is now well-known and has been subject to numerous recent
reviews (Lynch and Pedersen 2016; Davenport et al. 2017). Moreover, we now
understand many of the lifestyle changes that are limiting or corrupting the transmission of the microbiota from family to child. These factors include caesarean
deliveries, lack of breastfeeding, inappropriate hygiene that limits mother–child
transmission, poor diet, and antibiotics during pregnancy and infancy. This last
point is particularly worrying. The relative risk of obesity is linearly related to the
332
G. A. W. Rook