You can’t spend all time eating and then reproduce but then you can not reproduce and pay the heavy costs of child rearing without energy. Humans as well as all life on earth usually have to play a balancing act between getting enough calories out of the earth and then investing effort into reproduction. The first is the agricultural revolution of around 13k years ago and the second was the industrial revolution around 200 years ago.
However there have been two changes over the past 13000 years that have changed human lives too fast for our genes to “keep up”. We adapted to warming and cooling by very gradually growing hair and losing hair. We adapted to the dwindling forests over millions of years by walking out on the plains. The main drive of the book is that while there were an number of things that we evolved to adapt to certain climate and food situations (such as bipedalism, larger brains, longer child weaning times, hands and so forth) that the change in our situations over the past 13000 has developed far faster than our bodies can adapt. We live longer, have low infant mortality, spend relatively small amounts of the day ensuring we have enough calories and are unique in the animal kingdom in having offspring that are completely helpless until the age of about 18 years. However what is clear that over the past few million years where humans speciated from common ancestors share with chimps and other apes “we” have adapted to life on earth in a way that is fairly unique. Now it is silly to suggest that somehow humans are at the top of some sort of evolutionary order (if you measure success by the amount of biomass a species occupies on earth then ants win by some margin). What events happen and what adaptions we made such that now, 4.5bn years on from when the Earth was formed (or about 6000 years depending on what books you read) why humans seem to fair well at survival. Leiberman is an evolutionary biologist which means his area of study is about “why” humans are the way they are. That book was responsible for the sale of millions of pairs of latex foot gloves at £100+ a go. You may remember Daniel Lieberman from such books as “Born to Run”, a book that many a runner (including myself) put down and immediately vowed to eat only turnips and run barefoot.