As the title kind of intimates, On Deep History and the Brain is a short prospectus for two (related but distinct) historical projects: deep history (extending history into "prehistory") and neurohistory (history through the lens of neurochemistry). The first 100 pages focuses not on deep history itself, but the intellectual hangups that have kept it from being the norm. It's a historiography of the history/prehistory divide, tracing it from Biblical truth to rearguard action against geological revealed deep time and evolution to the lingering bias built into the discipline against methods not focused on written records and especially the subjects that demand such methods.
He builds this story in more detail than he perhaps needed to, but on the other hand, I'm constantly incredulous about this stuff, so maybe it is necessary. From the point of view of a modern academic, the premises of deep history are insultingly obvious: humans are animals subject to ecological rules, and the flow of history follows the logic imposed by those limitations, locating causal factors in blindly selected adaptations to changing conditions rather than intentional design and leadership. In a sense, it can't help but feel like Smail is ego-stroking us for the blessing of being born in the enlightened present, though it isn't framed that way of course. I'm just constantly astonished at how long the great man narrative approach to history persisted (he calls it the Bad King John approach, which was new to me), how long they clung to the specter of free will and intentionality. I've been reticent about really believing this idea was widespread for so long, since it reflects to poorly on so many historians, but Smail establishes a pretty solid case here.
In breaking down the barrier to deep history (ie, abolishing the concept of "prehistory"), he also eloquently makes the case for a history throughout time based on a broader body of evidence, focused on processes that may not be apparent to historical actors. Of course, this is a less pressing point, and it's more preaching to the choir than advocating deep history, perhaps. But it's still fun. Instead of giving an example, Smail details a line of research based on the eco-evolutionary logic of deep history that gives an understudied handle stretching across the divide into the deep past: neurochemistry.
Smail's idea of neurohistory walks a weird line between insight and tautology. The idea is that material circumstances and, especially, cultural patterns, shape our neurochemistry in discernible ways. Men in the South have measurably higher stress responses to offense due to the masculinity norms they were raised with. Women in Victorian England were, he speculates, physiologically more prone to fainting than women in other time periods, because their socialization was written in their endocrine system. That idea appeals to me, though extending it into history risks a lot of misleading analogy and speculation.
But his more general point is that most economic activity is driven by goods and activities that modulate human neurochemistry. I think it takes a careful thinker, and perhaps a particular case, for this to be meaningfully different from "people do things because they feel good." After all, the idea that some human activities are pleasurable but not necessarily adaptive is nothing new. This is maybe a bit unfair; unpacking the ways that social change is driven by new opportunities in mood-altering might offer more insights in the particulars than the observation that people do it reveals in general. It might, though Smail (perhaps in his intense aversion to evolutionary psychology?) doesn't make much of this, offer a way to chain histories of production to the evolutionary history of the human species (or its domesticated partners).
I'm a big fan of the way of thinking Smail advocates here, and he makes the case eloquently and concisely. It's perhaps not necessary reading if you're already convinced, though.