and walls were plastered with mixtures of mud, animal hair, and dung, while roofs
were often thatched. The microbiota of such homes was similar to that of the natural
environment with which we co-evolved, and harmless to humans. In contrast, the
modern home, built with plastic, concrete, and biocide-treated timber and plasterboard can become toxic (sick building syndrome) when damp or deteriorating,
because an unusual microbiota appears that has nothing to do with the microbiota
of our evolutionary past. The secondary metabolism of such organisms can be toxic
to humans (Andersson et al. 1998; Sahlberg et al. 2010). However, we can benefit
from measures that transport the microbiota of the natural environment into our
modern homes. One way of achieving this is to keep cats or dogs (Fujimura et al.
2010). A large Danish study found a dose–response relationship between the number
of household cats and dogs during the first year of life and reduced manifestations of
asthma, allergic rhinoconjunctivitis, or eczema) (Hesselmar et al. 2018). Similarly,
the more the microbiota of the home resembles that of the farm environment, the
lower the risk of asthma in children (Kirjavainen et al. 2019). But direct contact with
the natural environment may be a better way to enhance intake of organisms from
air, soil, and water via the airways, gut, and skin, as discussed below.
18.3.1 Air
When total numbers of organisms in air were counted (i.e. not only the cultivable
ones) levels of 10
2 /litre or more were regularly encountered over a grassy field on
clear sunny days, and estimates approaching 10
3
/litre have been reported above
shrubs and some grasslands (reviewed in Burrows et al. 2009). The air in facilities
housing agricultural animals can contain still higher numbers, reaching 10
4
–10
5
archaea and bacteria in every litre (Nehme et al. 2009). Since the average adult
breathes about 11,000 l of air each day, the input over a 24 h period can be anything
from 10
6 to 10
9 organisms, and even 10
10 if working hard and breathing heavily in
some environments, but the issue is complicated by the fact that deposition in the
airways depends on particle size. Large particles tend to impact on surfaces in the
upper airways, such as the nasal turbinates while small particles may enter the more
distal airways and interact with host tissues by diffusion. The microbial diversity of
air is comparable to that of seawater, soil, and the human gut, but in a recent study
only 9–17% of the airborne microbial sequences were identifiable (Gusareva et al.
2019). Thus, we really do not know what organisms are encountered via the airways,
though tropical air contains a relatively higher proportion of fungi with only traces of
phage and archaea (Gusareva et al. 2019). Clearly, soil organisms can enter the air
via dust in dry conditions, but we now know that raindrops impacting soil cause tiny
explosions of soil organisms to enter the air, and organisms are always present in the
air we breathe (Joung et al. 2017). They will also settle on the food that we eat.
18 Darwinian Medicine: We Evolved to Require Continuing Contact with the. . .
335
were often thatched. The microbiota of such homes was similar to that of the natural
environment with which we co-evolved, and harmless to humans. In contrast, the
modern home, built with plastic, concrete, and biocide-treated timber and plasterboard can become toxic (sick building syndrome) when damp or deteriorating,
because an unusual microbiota appears that has nothing to do with the microbiota
of our evolutionary past. The secondary metabolism of such organisms can be toxic
to humans (Andersson et al. 1998; Sahlberg et al. 2010). However, we can benefit
from measures that transport the microbiota of the natural environment into our
modern homes. One way of achieving this is to keep cats or dogs (Fujimura et al.
2010). A large Danish study found a dose–response relationship between the number
of household cats and dogs during the first year of life and reduced manifestations of
asthma, allergic rhinoconjunctivitis, or eczema) (Hesselmar et al. 2018). Similarly,
the more the microbiota of the home resembles that of the farm environment, the
lower the risk of asthma in children (Kirjavainen et al. 2019). But direct contact with
the natural environment may be a better way to enhance intake of organisms from
air, soil, and water via the airways, gut, and skin, as discussed below.
18.3.1 Air
When total numbers of organisms in air were counted (i.e. not only the cultivable
ones) levels of 10
2 /litre or more were regularly encountered over a grassy field on
clear sunny days, and estimates approaching 10
3
/litre have been reported above
shrubs and some grasslands (reviewed in Burrows et al. 2009). The air in facilities
housing agricultural animals can contain still higher numbers, reaching 10
4
–10
5
archaea and bacteria in every litre (Nehme et al. 2009). Since the average adult
breathes about 11,000 l of air each day, the input over a 24 h period can be anything
from 10
6 to 10
9 organisms, and even 10
10 if working hard and breathing heavily in
some environments, but the issue is complicated by the fact that deposition in the
airways depends on particle size. Large particles tend to impact on surfaces in the
upper airways, such as the nasal turbinates while small particles may enter the more
distal airways and interact with host tissues by diffusion. The microbial diversity of
air is comparable to that of seawater, soil, and the human gut, but in a recent study
only 9–17% of the airborne microbial sequences were identifiable (Gusareva et al.
2019). Thus, we really do not know what organisms are encountered via the airways,
though tropical air contains a relatively higher proportion of fungi with only traces of
phage and archaea (Gusareva et al. 2019). Clearly, soil organisms can enter the air
via dust in dry conditions, but we now know that raindrops impacting soil cause tiny
explosions of soil organisms to enter the air, and organisms are always present in the
air we breathe (Joung et al. 2017). They will also settle on the food that we eat.
18 Darwinian Medicine: We Evolved to Require Continuing Contact with the. . .
335