Where is the logic?

Is considered, that the human brain consumes a huge amount of energy. When we think intensely, this organ eats up to a quarter of the resources. According to modern anthropologists and evolutionists, brain volume, at the time of its appearance, Homo Sapiens had more, than modern man, and then it started to decrease, due to resource savings, and therefore we are now rapidly becoming stupid. This point of view can be found in the interviews and writings of Savelyev, Markova, and so on. This is what seems strange to me, several tens of millennia ago, in the most brutal, as is commonly believed, struggle for existence, and as a result of saving resources, “внезапно” there is a view, with a huge, and a creepy energy-consuming organ, the need for which at that moment (in this form) крайне сомнительна. Why so irrational, from the point of view of nature, поступок. For the then existing way of life, over and above, something simpler would be enough. But what is most interesting, as the amount of knowledge increases, Information, навыков, the gradual advent of an era of food abundance, the brain began to shrink and degrade, And we are, pier, we are rapidly becoming stupid … although it should have been the other way around (I still remember futuristic pictures, where the man of the future was portrayed as such a tadpole). Something doesn't add up.