I have not had a chance to go through all the details of this paper, but University of Zurich PhD student, Ana Navarrete (along with Carel van Schaik and Karin Isler), has a paper out in Nature this week that is likely to draw a lot of attention. In the paper, the authors attempt to test the “Expensive Tissue Hypothesis” by looking at relative brain and organ mass across a large range of species. Briefly, the expensive tissue hypothesis is based on several observations put together by Leslie Aiello and Peter Wheeler more than 15 years ago (see below for original reference). They noted first, that humans are highly encephalized, meaning we have a large brain relative to our body size. Second, brain is a metabolically expensive tissue, meaning it uses a large amount of energy per unit mass (thinking really is tiring…). Finally, basal metabolic rate, the rate at which our bodies produce energy, is highly conserved when you control for body size across taxa. So humans have a big brain for something our size, but do not seem to produce any additional energy to fuel that brain. Instead, what Aiello and Wheeler suggested is that we experienced an energetic tradeoff at some point in our evolutionary past, giving up one kind of expensive tissue for another kind (brain). Comparing across taxa, Aiello and Wheeler identified our digestive tract as the likely tradeoff, an idea that makes a lot of sense when you compare the dietary quality and amount of external food processing humans engage in relative to other organisms.
The expensive tissue hypothesis has been an important concept in paleoanthropology ever since, particularly in discussions centered on the origins of the genus Homo and transition to a diet that involves more animal products (fat + protein) obtained through hunting and/or scavenging. The concept is a great example of clever observation and integration of big picture (how do organisms produce and use energy) and little picture thinking (how did human brains get so big?). But I have always felt the concept is a better generator of hypotheses than a complete story. But, perhaps surprisingly, the idea has not been subject to a huge amount of testing. Instead it has largely been incorporated into a lot of mainstream thinking about human evolution.
What Navarrete’s research suggests is that the story is less about tissue-specific energy consumption and more about fat. Navarrete spent what I assume was a great deal of time dissecting, measuring and weighing the carcasses of animals. What she found was that brain size was not negatively correlated with digestive mass, but was negatively correlated with adiposity (fat). The more body fat organisms had, the smaller their brains tended to be relative to their size. The authors propose this represents two solutions to the problem of acquiring the basic energy to survive. One solution is to store it and carry it around with you in the form of fat. The other is to have a bigger brain, presumably giving you more behavioral capacity to creatively acquire nutrients.
I hope this article spurs more research into both of these hypotheses. Neither, it turns out, is actually that easy to test as both fat retention and tissue-specific energy usage are dynamic processes. Throughout a year, season, or lifetime, many organisms cycle through high-fat and low-fat time periods. An organisms adiposity, in other words, is not a fixed component (although it might have strong modal tendencies on a species level). The physiology and energetics of tissue function are also quite variable. My own suspicion is that both hypotheses likely are important in the story of human evolution. But hopefully, there will be much more to come…
1. Ana Navarrete, Carel P. van Schaik, Karin Isler. Energetics and the evolution of human brain size. Nature, 2011; DOI: 10.1038/nature10629
2. Leslie Aiello, Peter Wheeler. The Expensive Tissue Hypothesis: The brain and digestive system in human and primate evolution. Current Anthropology, 1995, 36(2):199-221; doi:10.1086/204350