  Why did the emergence of our species require a timescale similar to the
entire habitable period of our planet? Our late appearance has previously been
interpreted by Carter (2008) as evidence that observers typically require a
very long development time, implying that intelligent life is a rare
occurrence. Here we present an alternative explanation, which simply asserts
that many planets possess brief periods of habitability. We also propose that
the rate-limiting step for the formation of observers is the enlargement of
species from an initially microbial state. In this scenario the development of
intelligent life is a slow but almost inevitable process, greatly enhancing the
prospects of future SETI experiments such as the Breakthrough Listen project.
