EQs are handy because they can be calculated for living organisms from brain size data and from fossil organisms from endocranial volume measurements. Furthermore, although encephalization is a direct measure of brain size it is correlated with increases in brain complexity. So, using EQ, we can capture a general estimate of intelligence (which is a function of brain size and complexity) across a wide range of species.

Our understanding of the evolution of intelligence has been based on the common presumption that increased intelligence has been actively selected for over evolutionary time. This certainly appears to be the case. The earliest unicellular organisms on Earth did not possess nervous systems. The first multicelled animals (metazoans) developed simple nervous systems about 560 million years ago. More recent groups, such as vertebrates, have even larger brains and suggested a trend. Consistent with this pattern is that the most encephalized animal, modern Homo sapiens, arose only 500,000 years ago.

The evidence indicates that both maximum and mean intelligence has risen over the history of life on Earth. However, two types of mechanisms could account for this pattern. If earlier organisms arose near a lower limit on brain size, then, as diversity increased, mean brain size could only increase along with it. Brain size, in John Maynard Smiths words, had "nowhere to go but up." We term this mechanism a passive trend because it does not imply any active selection for increased intelligence. In contrast, an active or driven trend involves an upward tendency toward increasingly higher encephalization levels induced by natural selection. (See second image)

The study of whether encephalization trends are passive or actively driven is highly relevant to questions posed by astrobiology and SETI. If trends in intelligence are driven, then less encephalized species will evolve into higher encephalized (more intelligent) species more frequently at a faster rate than otherwise. Therefore, the expected number of highly encephalized species at any given time will be greater than if no such tendency existed. Further, larger numbers of highly encephalized species increases the probability that at least one such species will survive extinction, or in other words, that intelligent life will be continuously present somewhere.

Encephalization Trends in Cetaceans

In 2002 my colleague Dr. Daniel McShea (a biologist at Duke University) and I were awarded a grant from the SETI Institute Center for the Study of Life In The Universe (LITU) to study trends in cetacean brain evolution. Cetaceans are highly encephalized, possessing EQs that range very close to that of humans and higher than that of other mammals. Yet cetaceans havent shared a common ancestor with primates for over 85 million years. As a result, their brains are very differently organized than primate brains. Therefore, cetaceans afford us the opportunity to examine a highly elaborated brain that has taken a very different evolutionary trajectory from our own.

Our method involves using estimates of brain and body weight from fossil and modern cetaceans. These data were collected as part of a National Science Foundation (NSF) grant awarded to Dr. Mark D. Uhen of the Cranbrook Institute of Science and me. The LITU-funding has enabled us to take our data collection a step further and address questions about trend mechanisms in the pattern of encephalization that occurred throughout cetacean evolution. We are completing our analyses and plan to publish our findings shortly. We hope to further extend our study of encephalization trends to a wide array of taxonomic groups such as carnivores, ungulates, birds, and cephalopods so that we can address questions about evolution at a large cross-taxonomic scale.

With our work we intend to help shift the focus of astrobiological questions about the evolution of intelligence from armchair speculation to empiricism, quantitative analyses, and scientific objectivity. More generally, we hope to show that questions about evolutionary forces at the largest scale, all animal life over most of its history, are addressable in a rigorous way.