Surely the definition of what a paleolithic/evolutionary diet is one that our ancestors ate during our species evolution. However working out what they ate is not a simple task. Some types of evidence survive better than others. Plant foods rarely survive. Some recent examples that may be of interest to people here: I apologize that some are subscription only.
http://www.ingentaconnect.com/content/ap/as/2002/00000029/00000007/art00743 Neanderthals collecting grass seeds.
http://infolib.hua.edu.vn/Fulltext/ChuyenDe2009/CD221/42.pdf Neanderthals eating legumes, particularly wild lentils. Also acorns and pistachios.
http://www.sciencemag.org/cgi/content/abstract/326/5960/1680 Africans 105,000 years ago eating sorghum grass seeds.
http://www.sciencemag.org/cgi/content/summary/324/5927/588-b Early humans using their stone tools to slice up tubers as well as meat.
Australian Aborigines have traditionally gathered many wild seeds and grains. http://en.wikipedia.org/wiki/Bush_bread
I do not wish to imply that the everyone should start eating bread or that these were staple foods in early human diets. Simply suggesting that hunter-gatherer diets throughout our evolutionary history have probably been more varied and inclusive than some people today imagine them to be?
I definitely think people have a simplistic view of paleolithic diets. There's a world of difference between "NEVER consumed EVER" and "was not a staple or evolutionarily significant food/not optimal/not necessary/potentially damaging". I also think the basic logic of eating the same way as whatever aggregate HG metaphor-myth you choose to construct is flawed, so these sorts of studies are more academic. Paleo is a starting point, a heuristic, a hypothesis generator. It guides you through self-experimentation through food elimination. It should never be taken as a hard-line, inviolate principle. Some people probably tolerate grains just fine - more power to them.
I do find it somewhat hard to believe that grass seeds provided enough calories to become a staple food. It seems plausible that humans ate whatever they could whenever they could through any means they could (starvation is a great motivator) but even so, just how big was their flour making industry, exactly? They clearly valued it enough to invest time and energy in making it, but how much? Grains on stone tools shows that they were grinding grains, certainly, but there's a pretty big difference between a lump of sourdough every now and then and bread as the basis of a diet.
Eating Paleo NYC did a paleo dinner last week and apparently the chef misunderstood some of our preferences and served us what amounted to about 30 fava beans under our venison. Then they served us some masa with our pig's head. Oh well...we all ate them both! If they had served us creme puffs and candy we probably would have said no thanks, but these little blips were not the entrees, they were just sides, and small amounts of them are probably evolutionarily appropriate.
Plus, we don't live in a vaccuum. One of the reasons I object to diets like zero carb and veganism is that people miss out on experiencing other cultures. One of the reasons we ate the fava beans and the masa was because on a previous outing we went to a Korean restaurant and pretty much accidentally insulted the elderly owner because we turned down bowls of rice. She was so upset that she insisted we would be hungry and brought out tofu. Honestly, the rice was probably more harmless than the tofu, so we would have been better off respecting her culture and cuisine and eating a few spoonfuls of rice.
Last year I grew pigweed and I've also harvested wild rice...harvests are so so so tiny compared to growing domestic corn or wheat. I eat grains at least once a week, but I always treat them as a side dish and serve them traditionally prepared (soaking, fermenting) in small amounts. It's quite amusing though...some anthropologists (particularly Katherine Milton) seem to want to believe that prehistoric people ate more plants than they actually did and grasp at tiny bits of evidence and then say "PPLANTS ARE SOOOOOO IMPORTANT 111!!! EVERYONE B VEGAN!"
Treating them as a staple probably is evolutionarily inappropriate, but treating them as a side dish isn't going to harm you unless you are already sick.
Tubers are a different and more complex matter that perhaps deserve their own topic.
I love the paleosphere, even the fact that we ask if we're becoming dogmatic is an insurance against it. We confront contradictory information, we don't just put our fingers in our ears and shout 'LALALALA' like other groups.
I think we can acknowledge that populations eat things that we avoid (traditionally prepared of course) and still be relatively healthy. We still choose to hedge our bets and avoid them, such is our prerogative.
"It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
And these are a FAR FAR cry from what grains are today or even 50 yrs ago, and getting worse with the lies of Monsanto playing God and screwing with nature (let alone sueing farmers). Early grains were never the norm but you will eat bark to stay alive in hard times and seem to be more like seeds.
soaking grains and seeds allowws the 'protective' nature of the seed to become harmless.
There use of seed grains is not white trash white beard and cheerios.