Why did brains get so big? There are a number of obvious reasons. One of them, of course, is for culture and for cooperation and language and various other means by which we can interact with each other, and certainly those are enormous advantages. If you think about other early humans like Neanderthals, their brains are as large or even larger than the typical brain size of human beings today. Surely those brains are so costly that there would have had to be a strong benefit to outweigh the costs. So cognition and intelligence and language and all of those important tasks that we do must have been very important.
Until extremely recently, you couldn't live, you couldn't survive as a human being without being an endurance athlete. Not just hunting and gathering requires athleticism but also being a farmer. Subsistence farmers have to work extremely hard. Until the invention of industrialized machinery, farmers had to work even harder than hunter-gatherers, often spending many thousands of calories a day. They have to dig ditches and throw vast quantities of hay into bales and they have to schlepp stuff all over the place. Farmers had to work brutally difficult, hard, exhausting lives. It wasn't until, again, the invention of new technologies such as domesticating animals or even more recently machinery such as the internal combustion engine, that farmers were able to live non-grueling lives.
To be true to your nature, don't let that desk job take over completely. Sit on a yoga ball, and go for a hike (or run) now and then.