A decade ago, very little was known of the inventory of microbes that inhabit different parts of the human body, and how they assemble into communities of varying levels of complexity. 8
The intestine, or more precise, the gut mucosa of the human's body hosts an extremely rich microbial ecosystem, and up to one thousand different bacteria, in total 100 trillion, 10^14, even up to ten times more than human cells and as much as 100 times as many genes as our genome 7. Each milliliter of the large intestine holds approximately 10^11 microbial cells, besides viruses and fungi, known collectively as the microbiome, where they live in a manner of mutual benefit. Without it, we would not be able to survive. It begins to affect our body even before we are born. The microbiome is extremely important for the immune system, heart, weight and many other aspects of health. Each type has a different function. They weight all together roughly as much as the human brain. Together, they function as a distinct organ in the human body. They supply essential nutrients, synthesizing vitamin K, aiding in the digestion of cellulose, and promoting angiogenesis and enteric nerve function a. 2
The Human gastrointestinal microbiota has following key roles 6:
- directly defending against pathogens
- fortifying host defense by its role in developing and maintaining the intestinal epithelium and inducing antibody production there
- metabolizing otherwise indigestible compounds in food
- training the developing immune system
- role in the gut-brain axis, which can alter minds and behaviors through the central nervous system. Maintaining gut microbioma symbiosis is important for retaining healthy central nervous system functions
Gut bacteria are essential for the transformation of natural compounds (e.g., lignans b) to perform their bioactivities. Lignans are present in a wide range of foods, such as flaxseed, vegetable, fruit, and beverages. Lignans afford protection against cardiovascular diseases, hyperlipidemia, breast cancer, colon cancer, prostate cancer, osteoporosis and menopausal syndrome, dependent on the bioactivation of these compounds to enterolactone (ENL) and enterodiol. Gut bacteria are required for the production and bioavailability of these enterolignans.
Gut bacteria also play an essential role in the metabolism of isoflavones, which have protective activity in breast cancer, prostate cancer, cardiovascular disease, osteoporosis, and menopausal symptoms
Bifidobacteria for example digest the sugar of mother's breast milk and important for growth. Others digest fibers, and produce short-chain fatty acids, important for gut health. Fibers prevent gain of weight, diabetes, heart disease and the risk of cancer. Further, they help control the immune system by communicating with immune cells and affect the central nervous system, which controls brain function.
Bacteroidetes and Firmicute bacteria help in the metabolism of undigested food remnants.
Certain Bifidobacteria and Lactobacilli, which are found in probiotics and yoghurt, help seal gaps between intestinal cells and prevent leaky gut syndrome. 1
Gut microbiome plays an important role in promoting "good" HDL cholesterol and triglycerides. Further, it also helps control blood sugar, which can affect the risk of type 1 and 2 diabetes. The gut is physically connected to the brain through millions of nerves. Therefore, the gut microbiome affects brain health by helping control the messages that are sent to the brain through these nerves.
Alterations in flora balance can cause many diseases and affect the ability to digest food. An overabundance of some kinds of bacteria may contribute to inflammatory disorders, metabolites from certain members of the gut flora may influence obesity and colon cancer, inflammatory bowel diseases, Crohn's disease and ulcerative colitis, are chronic inflammatory disorders, Irritable bowel syndrome. Imbalances in the gut flora may cause allergy, asthma, and diabetes mellitus which are autoimmune and inflammatory disorders, obesity, metabolic syndrome, liver disease, cancer etc.
Understanding the human microbiome, its diversity, stability, and resilience and how these influence health is a remarkably hard problem for science. Of course, if homeostatic balance is required, the question arises, how this relationship of mutual benefit emerged in the first place. Since science is still in an infant stage to unravel these complex relationships, it permits plenty of room for evolutionary storytelling. But how do science papers actually answer this question ?
A recent paper, published in 2019, makes following observations:
If the mutation rate in the gut is similar to estimates in the laboratory (i.e., approximately 10−3 per genome per generation), then in each gram of material there will be 10^4 to 10^5 new mutant cells. Of these, only a fraction would carry alleles with local benefits to its bacterial host as it competes in the gut ecosystem. If this fraction is minute (i.e., 1 in 1 million mutations is of any good at any moment), then evolutionary change via this mechanism will be very slow. 9
Fact is, science has no answer to this relevant question yet. Another paper from 2018 notes: 10
A better understanding of the colonization process, and how it might vary among the species, is an important avenue for future work.
My remark: It seems to me, this is another example of all or nothing business. The symbiosis of the human gut and beneficial bacteria had to exist from day one. Of course, there is always room to say, the cooperation began naturally, no design required. That seems to me to be one more " just so" story, which lacks credibility.
a) The enteric nervous system (ENS) is a quasi-autonomous part of the nervous system and includes a number of neural circuits that control motor functions, local blood flow, mucosal transport and secretions, and modulates immune and endocrine functions. 3
b) The lignans are a large group of polyphenols found in plants 4 Polyphenols are a structural class of mainly natural organic chemicals characterized by the presence of large multiples of phenol structural units. 5