Solar energy trapped by plants is converted mainly into cellulose, that is indigestible by us, since we lack cellulose-degrading enzymes. Cellulose is degraded solely by microorganisms. Microbes and degradation products are ingested by protozoa. These in turn are ingested by higher live forms and so on. At the high end of this food chain are the animals that serve as our food.
Microbes are everywhere, and inhabit our skin and intestine.
The newborn baby receives its microbial flora from its mother, that already infects it at birth. Then come the facultative anaerobes, such as Escherichia coli and Streptococcus fecalis, that are followed by strictly anaerobic bacteria, e.g., Bacteroides. At the time of weaning, populations of obligate anaerobes become dominant. The development of our personal microbial community, known as primary succession, involves an ordered sequential change in the microbial populations of the community. Succession ends when the community consists of up to 400 different species, and attains homeostasis.