# Human evolution and gut flora ? **URL:** \ **Category:** Factual Questions\ **Created:** [April 14, 2014, 7:34pm UTC](https://boards.straightdope.com/t/human-evolution-and-gut-flora/686108 "2014-04-14T19:34:08Z")\ **Posts on this page:** 5\ **Page:** 2
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [April 16, 2014, 2:09pm UTC](https://boards.straightdope.com/t/human-evolution-and-gut-flora/686108/21 "2014-04-16T14:09:46Z")
> [@dofe](#): > > I don’t think the immune system plays a role in your digestive tract. This sounds counterintuitive, but your digestive tract is technically an “outside” surface. Think of your body as an elongated donut or a tube. The “gut” is actually just part of your skin, except its tucked inside the surface of the tube. [_Das ist nicht nur nicht richtig, es ist nicht einmal falsch!_](http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/gi_immune.html). – Wolfgang Pauli Stranger ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [April 16, 2014, 2:56pm UTC](https://boards.straightdope.com/t/human-evolution-and-gut-flora/686108/22 "2014-04-16T14:56:35Z")
> [@DSeid](#): > > **Exapno** I am not so sure though that that does not count as inheriting. Clearly most of an infant’s biome is from the infant’s mother and clearly we have co-evolved with our microbiomes over many many generations along with our microbiome also adapting in fairly short periods of time to the circumstances we present it with (high fiber or high fat, so on). > [@am77494](#): > > That was a good cite, and it explained the flora very well. Would you please explain why you think that the mom is not the significant contributor to the baby’s gut flora ? The article seems to say as much. I see the difference as a critical one between genetic and environmental. Internal flora is always picked up from the surrounding environment. In a human baby that is normally the mother but the critical difference is that the flora does not arrive through the umbilical connection but through chance encounters with surfaces during birth. As my linked article states, a completely different set of flora is encountered during a C-section. We undoubtedly have a good internal environment for the care and feeding of beneficial bacteria, but we can’t control which beneficial bacteria we encounter nor can we prevent encounters with harmful bacteria. > [@DSeid](#): > > One small example of that process: breast milk contains high amounts of this stuff called oligosaccharides. Babies cannot digest it. So what the heck is it doing there? It’s the preferred feedstock to the sorts of bacteria that benefit us long term, that’s what. We are evolved to give certain populations of bacteria what they need to thrive. And they in turn work in concert with each other, some existing off of the H2 that others produce. Other than the quibble that breast milk is not high in oliosacchardies - there are 7 g of lactose per ounce and only 0.5 g of oligosaccharides - this is true. Both of those types of carbohydrates do provide preferential environments for various genera of beneficial bacteria. Preferential is still different from necessary. Babies on formula or raised on other mammal’s milk can still thrive equally. > [@am77494](#): > > I am trying to understand this. I’d think the immune system will attack the non-beneficial bacteria and I asked the question if we have evolved to make the beneficial bacteria’s environment in the gut more hospitable ? The evidence says this is wrong. Go back to my earlier example of how lactose-digesting bacteria can prevent the symptoms of lactose intolerance. Some people, however, have lactose-\*fermenting \*bacteria which also break down the lactose, but not into usable simple sugars but into fatty acids and gases, which result the gas and bloating, the hallmark symptoms of lactose intolerance. There doesn’t appear to be any preference for one kind over the other; it’s a matter of your exposure and your diet. You can’t even say one is good and the other isn’t. If you didn’t have lactose in your diet they would sit there and not do anything; they’re neutral until the environment changes. Similarly, there are the multitude of bacteria that are implicated in traveler’s diarrhea. The most common one is the dread _E. coli_. Yet, locals can build up toleration to these bacteria and stay perfectly well while tourists are leaking from every orifice. How can this be generic? Viruses can also cause this effect. Are there any beneficial viruses? Even if there were it’s hard to reconcile a genetic adaptation to microbes that leaves out the worst killers. > [@](#): > > Agreed - what I am asking is if the degree of warmthness (temperature) and degree of moistness evolved to support the beneficial bacteria and hinder the non-beneficial ones ? I see no evidence of this. Bacterial effects depends on the environment. I don’t know of any evidence that non-beneficial bacteria are hindered, avoided, killed, segregated, or in any way lessened. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [April 16, 2014, 7:05pm UTC](https://boards.straightdope.com/t/human-evolution-and-gut-flora/686108/23 "2014-04-16T19:05:30Z")
“[Oligosaccharides](http://http://journals.lww.com/jpgn/Fulltext/2000/03002/Oligosaccharides_in_Human_Milk_and_Bacterial.3.aspx) are, collectively, the third largest solid constituent in human milk, after lactose and fat” … I consider that a major componant. Not sure what your comment about required has to do with the question. The simple fact is that we have evolved with the means to foster the growth of the bacteria that we want. Breast milk compostion is part of it. > [@](#): > > Journal of Pediatric Gastroenterology & Nutrition: > March 2000 - Volume 30 - Issue - pp S8-S17 > Oligosaccharides in Human Milk and Bacterial Colonization > Newburg, David S. > > … In the first year of life, a stable population of microflora is established in the human gut as a complex ecology that includes the intestinal mucosa and more than 400 interdependent species of bacteria. During this period, a child is vulnerable to enteric and other diseases, but breast-fed infants have a lower rate of morbidity and mortality than do artificially fed infants. The lower incidence of disease in infants while they are breast-feeding has been attributed to many causes: less exposure to contaminated water, better nutritional status, and psychological factors. Although these factors probably contribute, the most direct determinant of reduced disease incidence among the breast-fed seems to be factors in the milk itself. > This protection is thought to result from two major kinds of components in human milk: those that provide a prebiotic effect that enhances colonization by Bifidobacterium bifidum, and others that inhibit the colonization and growth of pathogens during this vulnerable period. > Three major classes of human milk components are known to inhibit pathogens: secretory antibodies; multifunctional agents, such as fatty acids and lactoferrin; and homologues of host cell surface glycoconjugates. Evidence is mounting that these homologues, consisting of glycoconjugates and oligosaccharides, have a major role. Glycoconjugates are molecules that have conjugated carbohydrate moieties (glycoproteins, glycolipids, glycosaminoglycans, and mucins); milk oligosaccharides have carbohydrate moieties conjugated to lactose. These milk carbohydrate moieties often share structural motifs with the cell surface glycoconjugates that pathogens adhere to when gaining entry to host target cells. Indeed, specific human milk oligosaccharides and glycoconjugates have been shown to block binding of specific pathogens to host cell glycoconjugate receptors … Yes, we still get exposed to bacteria if we do not go through a vaginal canal. Guess what? Mom wipe and are not always perfect about handwashing. Breasts and breast milk is full of the same bacteria. C-section and formula fed results in a different microbiome. There are other aspects of the gut such that a functional one is still selected but it is different. Thrive yes, thrive equally? No, there are measurable differences. Read the cites already given. You are simply mistaken that the human gut does not adjust to fight off pathogens include with immunologic mechanisms and to foster “good” bacteria. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [April 16, 2014, 9:13pm UTC](https://boards.straightdope.com/t/human-evolution-and-gut-flora/686108/24 "2014-04-16T21:13:50Z")
I think we’re arguing past one another. Of course breast milk is beneficial and I certainly advocate it for anyone capable of doing so. That’s still a different issue than the one I’m talking about. It’s hardly surprising that breast milk contains material to help the newborn. That is not relevant to the 95+% of one’s life when no breast milk is being consumed. The gut does not adjust itself to fight off pathogens as an adult. Heck, it doesn’t even do so as an infant given the number of pediatric illnesses that are pathogen-borne. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [April 16, 2014, 10:02pm UTC](https://boards.straightdope.com/t/human-evolution-and-gut-flora/686108/25 "2014-04-16T22:02:38Z")
Breast milk was just one example of how we have evolved to select for beneficial bacteria and keep pathogenic bacteria out. Other examples have also been given of how we have an environment that fosters the growth of beneficial communities across the life cycle. Of course it is not 100%. But you seem to be under this strange misconception that we passively take in whatever bacteria we happen to be exposed to. Such is not even remotely close to the case. There is an active process developing and maintaining a beneficial microbiome and the microbiome has been selected for in broad brushstrokes over evolutionary history, co-evolving with us. We get seeded with bacteria from mom and immediate family members. On the evolutionary scale that was always through exposure during birth and ongoing exposure afterwards. We then foster the growth of the communities we want with breast milk prepping it for other foods later and developing the gut immune system. The microbiome so prepped works with our immune systems to then keep pathogens from getting a foothold and repelling them when they do. Current microbiomes are considered by experts in the field to relatively impoverished ecosystems, secondary to the lack of fiber in our diets and exposure to antibiotics, especially early in life. The impact of that impoverishment is just beginnning to be understood. We hve evolved markers in different portions of the gut to encourage certain communities to prosper there and take advantage of them to do hosts of things for us. Those who developed the markers to promote benficial bacteria and to discourage pathogens lived longer and reproduced more; the bacteria that could thrive thusly seeded to next generations more. Co-evolution. [Previous page](https://boards.straightdope.com/t/human-evolution-and-gut-flora/686108.md?page=1)