I have decided to view human evolution in the larger context of primate evolution, because we share so many characteristics with other primates and have a genome 98 percent similar to that of our nearest relative, the chimpanzee. Since modern humans eat legumes, and humans share a common ancestor with chimps, if modern chimps eat legumes, this would suggest that probably the last common ancestor of humans and chimps also ate legumes.
So I decided to find out, do modern wild chimps eat legumes?
It only took a few internet searches to find that, indeed, non-human primates, including chimps, consume legume seeds, leaves, and flowers.
The most remarkable of the literature I have so far come across on this topic is a paper published in the American Journal of Primatology by Shoeninger, Moore, and Sept, entitled “Subsistence Strategies of Two “Savanna” Chimpanzee Populations: The Stable Isotope Evidence.” [1 pdf] In this paper, the authors report on Ugalla chimps, living “in open, grassy woodland habitats similar to those in which the last common ancestor of apes and humans probably lived.” These chimps consumed a diet very rich in fresh legumes, estimated at 50% of total food consumption, certainly a level requiring some level of physiological adaptation.
This puts fresh legumes in a different class from grains. So far as I know, we have no evidence of chimps consuming any significant amounts of immature grass seeds (grains).
|Green Peas. Source: Ecosalon|
Based on this type of evidence, it seems probable that fresh legumes were part of hominoid diets for millions of years before the advent of agriculture. This would give plenty of time for hominoid physiology to become adapted to regular intake of fresh legumes and their phytochemical constituents, and also provide an evolutionary pathway to the domestication of legumes.
I know many people feel worried about isoflavones with phytoestrogen properties affecting sexual development, function, and fertility. They have the idea that plants produce these compounds to disrupt the fertility of animals consuming them.
It is easy to think of the herbivore as the enemy of the plants it consumes, and vice versa, but grazing herbivores provide water, nitrogen, and minerals to plants via saliva, urine, and feces deposited in the field while grazing. Herbivore hooves also knead and soften the soil. The plants receive many needed services from their ‘enemies,’ not the least of which is a supply of carbon dioxide, without which they can’t live. The herbivores need the nutrients and oxygen the plants produce. Food plants and animals using them form a yin-yang pair, complementary and opposite, but if antagonistic, both sides fail.
If a plant slightly limits the fertility of an animal grazing upon it, this actually serves the animal species. Sure, some individuals may complain because they don't get the litters they want, but by keeping the animal numbers within limits, this reduces the chance that the animal population will overshoot its resource base and crash, while also increasing the amount of food/nutrients available for each individual animal, increasing the quality of life for the grazier. The plant helps the animal maintain a sustainable population size, and by grazing, the animal helps the plant maintain a sustainable population size. In the big picture, this is synergism, not antagonism.
The synergism and mutuality of plant-animal nutrition relationships is especially evident in human interactions with plants. When humans like a plant, usually because the plant helps them thrive and reproduce, the people take on the task of feeding, protecting, and promoting the reproduction of that plant. Humans help plants that help humans thrive, so plants that help humans have become among the dominant plant species on the planet.
When thinking about evolutionary plant-animal interactions, I feel it is important to realize that organisms adapt not only to ‘beneficial’ but also to challenging aspects of their habitats, if given enough time.
Let’s assume that at some point in the past, some herbivores were grazing on plants rich in phytoestrogens. Let’s also assume that, initially, the herd grazing on these plants does have reduced fertility. Nevertheless, within the herbivore herd a range of susceptibility to the phytoestrogens’ effects on fertility. That is, some of the animals may be rendered completely infertile, some will have reduced fertility in varying degrees, some will have no reduction in fertility, and it is possible that in some animals the increase of phytoestrogens will actually increase fertility.
If this process continues for several generations, gradually the animal population will move toward adaptation to the isoflavones. The animals resistant to the anti-fertility effects of the isoflavones will have more offspring than those not resistant. Eventually, the entire herd will have resistance to the effects of the typically encountered levels of isoflavones.
Now, let’s suppose that the mechanism of action of the isoflavones is to reduce hormone levels in the animals. In this situation, the animals resistant to the anti-fertility effects of these phytochemicals will be those who have an endogenous production of hormones high enough to counter the negative effects of the phytoestrogens. Over several generations, the evolutionary result will be a species adapted to a phytochemical drain on its endogenous hormone production by virtue of a higher endogenous output of hormones to compensate for the losses induced by the phytochemical.
Now, if you take this species off of the diet to which it is adapted, removing or greatly reducing the ‘hormone disrupting’ phytochemicals, the animal’s usual hormonal output might be excessive. As a consequence, the animal might develop disorders due to excessive levels of its own hormones. Adding the phytochemicals back to its diet will reduce those hormone levels, producing a more balanced physiology, because the animal is genetically adapted to a diet containing chemicals that ‘disrupt’ its hormones. It may actually need the ‘hormone disrupters’ to maintain hormone balance.
|Edamame. Source: Dried-edamame.com|
I suggest that this may provide part of an evolutionary explanation for the growing body of research suggesting that consumption of legumes and other plant foods containing phytoestrogens may have positive effects on human health.
I discussed here some research that supports the idea that plant-rich diets and specific whole plant foods can reduce the excessive sex hormone levels present in premature menarche, premenstrual symptoms, menstrual pain, polycystic ovary syndrome, hirsutism, menopausal syndrome, and reproductive system (breast, ovarian, etc.) cancers in women.
Tham et al of the Stanford Center for Research in Disease Prevention and the Department of Medicine discuss the growing evidence for potential health benefits of dietary isoflavones and lignans, two types of phytoestrogens including prevention of cardiovascular disease, promoting bone health, and regulating hormone levels across the life cycle, in both men and women, to prevent sex hormone-linked reproductive system cancers. 
World-wide patterns of human population growth seem to lend little support to the idea that phytoestrogens make people infertile. Historically, growth rates have been luxuriant in nations consuming more plant-based diets (India, China, Asia in general) rich in phytoestrogens.
Legume proteins may also have unique benefits. For example, multiple studies have shown that substituting soy protein for animal protein might improve kidney function in type II diabetics with nephropathy [3, 4, 5, 6, 7, 8]. This may not be a property unique to soy, but an effect of legume protein versus animal protein, due to legume proteins having a different ratio of amino acids. It certainly does not indicate lack of adaptation to legume proteins.
I find it hard to fit this data into an picture of human evolution that considers legumes discordant with human biology, but it makes sense in a view that includes legumes among human ancestral foods.
Lignans are another type of phytoestrogen. As shown in this table, lignans occur in fruits and vegetables as well as seeds, nuts, legumes, and grains. Although the seeds typically have the highest concentrations, sweet potatoes, carrots, asparagus, and garlic have levels comparable to pinto beans, peanuts, and several grains.
Legumes like clover naturally occur in grasslands, and farmers grow clover as part of their pastures and fodder for ruminants. Consequently, products from either pasture- or grain/legume-finished animals also can contain phytoestrogens, although in lesser amounts than in plants. Hence, human ancestors probably would have gotten exposed to these compounds through eating wild game meat as well as plants.
Of course, as with every other item we ingest, dose and context affects outcome. Nature never delivered isoflavones in concentrated pills or isolated legume proteins, absent counter-balancing compound present in the whole foods, nor did it give isoflavone-rich soy formula (based on soy protein isolate) to human infants. Obviously, substituting soy infant formula for human breast milk is discordant with human biology.
Now on to one of America's favorite beans.
|Coffee Bean. Source: Whos3d3n|
Did you know that coffee supplies the same isoflavones found in soybeans, albeit in smaller amounts?
"This paper reports the isoflavone contents of roasted coffee beans and brews, as influenced by coffee species, roast degree, and brewing procedure. Total isoflavone level is 6-fold higher in robusta coffees than in arabica ones, mainly due to formononetin. During roasting, the content of isoflavones decreases, whereas their extractability increases (especially for formononetin). Total isoflavones in espresso coffee (30 mL) varied from 40 μg (100% arabica) to 285 μg (100% robusta), with long espressos (70 mL) attaining more than double isoflavones of short ones (20 mL). Espressos (30 mL) prepared from commercial blends contained average amounts of 6, 17, and 78 μg of genistein, daidzein, and formononetin, respectively. Comparison of different brewing methods revealed that espresso contained more isoflavones (170 μg/30 mL) than a cup of press-pot coffee (130 μg/60 mL), less than a mocha coffee (360 μg/60 mL), and amounts similar to those of a filtered coffee cup (180 μg/120 mL)."