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While many habitats have plants, fruits, nuts and berries available for consumption by humans walking by, most places (if any?) seem to require significantly more effort than simply picking our food from trees and bushes in order to survive.

Borrowing @BryanKrause's words in a comment below:

"Why don't you find enough things like broccoli, green peppers, almonds, and wheat to support a human population when you are walking in the woods/natural fields?"

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    $\begingroup$ because except in the case of fruit plants don't benefit from being eaten so they have evolved a wide range of defenses. You might as well ask why gazelle just don't lie down and let lions eat them. $\endgroup$ – John Apr 18 '17 at 2:21
  • $\begingroup$ @John But gazelles graze grass, why haven't either we evolved to eat such abundant food or why haven't any plant species filled that niche? I guess this is a set of rather complex and unanswerable questions… $\endgroup$ – trmdttr Apr 18 '17 at 20:51
  • $\begingroup$ Because there are already many grazers in africa, and having evolved from omnivores our ancestors could not compete with them. Grass may be abundant but it it rather difficult to digest grazers need to be specialized. Why haven't plants evolved to fill what niche? $\endgroup$ – John Apr 19 '17 at 0:22
  • $\begingroup$ I meant the niche of being easily available food for humans? @John I guess these questions really are unanswerable… $\endgroup$ – trmdttr Apr 19 '17 at 14:12
  • $\begingroup$ The questions are perfectly answerable if, asked clearly. $\endgroup$ – John Apr 19 '17 at 21:24
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Define "easily available to human", lots of fruit are perfectly edible to humans. Human have only become widespread in the last few million years so there is not enough time to see things specialized in humans via natural selection. Especially since we are both generalists and for most of the world completely new factors in the environment.

But there are plants that have evolved to be eaten by humans. Wheat, rice, and, maize have done amazingly well by having the right factors to be useful to humans, they are also some of the most common plants on earth now thanks to us. Artificial selection is still evolution, it's just not natural selection. Humans are generalists and its hard for natural selection to push things to specialize in edible symbiosis with a generalist, especially in as short a time as humans have been around, but artificial selection can do it just fine becasue it is faster, provided the plant in question survives us long enough.

Being useful to humans is also a bit of a mixed bag sometimes it means we spread you over the whole world, such as wheat and dogs, but sometime is means harvesting to extinction like Silphium. Then you have things like avocado which almost went extinct becasue humans killed off their parter animals and almost nothing else could eat them, then we invented farming and decided we liked them and brought them back from the brink. Human intelligence and cultural information sharing means capabilities and technology change so much faster than the speeds natural selection tends to work at, most things just have not had the time to really adapt to how fast we changed the world, unless we take an interest in forcing them to.

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Humans prefer the very tenderest of plants as found in shops, which only grow abundantly when there is a lot of rain and warmth: salads, roots, tubers, fruit. Other animals are not so fussy, they have more stomachs and more grinding teeth. So plants that are palatable to humans have very few chemical and physical protections, and are good food for all herbivores. That's why the majority of plants have chemical and physical defenses, cellulose that makes them stringy, hair, tannins and so forth. plants favored by humans are favored by insects and mammals too. harsh habitats that favor slow growth have very few fast growing and low cellulose plants, i.e. mediterranean, where everything is strawy and has spines and strong resins. In cold habitats, juicy low cellulose plants can't grow fast enough to gain advantage by rapid low cellulose growth. In arid habitats, insects and animals covet juicy salad type plants, and they are eaten fast and also grow slower. Only in lush green habitats, i.e. tropics and in temperate climates can indigenous people go for a 10 minute walk and bring home malva, palms, thistle stems, reed roots, vine, and make a salad in a very fast time.

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