Carter's theory assumed set timescales for two processes: the life cycle of a star and the emergence of complex life. By statistically combining the two Carter concluded that complex life takes longer to emerge than the life-friendly duration of most stars -- with the implication being that intelligence is excruciatingly rare in the Galaxy and we may be alone.
Not satisfied with this conclusion, Ćirković and colleagues Branislav Vukotić and Ivana Dragićević are now disputing these assumptions. In their Astrobiology paper, "Galactic Punctuated Equilibrium: How to Undermine Carter's Anthropic Argument in Astrobiology," they contend that there is no reason to assume life evolves only gradually. They argue life could evolve in fits and starts - mirroring an evolutionary theory called punctuated equilibrium.
The abstract of their paper reads,
Our approach is based on relaxing hidden uniformitarian assumptions and considering instead a dynamical succession of evolutionary regimes governed by both global (Galaxy-wide) and local (planet- or planetary system–limited) regulation mechanisms. Notably, our increased understanding of the nature of supernovae, gamma-ray bursts, and strong coupling between the Solar System and the Galaxy, and the theories of “punctuated equilibria” and “macroevolutionary regimes” are in full accordance with the regulation-mechanism picture. The application of this particular strategy highlights the limits of application of Carter's argument and indicates that, in the real universe, its applicability conditions are not satisfied. We conclude that drawing far-reaching conclusions about the scarcity of extraterrestrial intelligence and the prospects of our efforts to detect it on the basis of this argument is unwarranted.In plain English, the conditions in the Universe required for the emergence of intelligent life have only recently been established (in cosmological scales). Prior to 'recent times', universal mechanisms were in place to continually thwart the evolutionary development of intelligence, namely through gamma-ray bursts, super novae and other forms of nastiness. Occasional catastrophic events have been resetting the "astrobiological clock" of regions of the Galaxy causing biospheres to start over. "Earth may be rare in time, not in space," they say. They also note that the rate of evolution is intimately connected with a planet's environment, such as the kind of radiation its star emits.
This is why the authors reject a strict uniformitarian approach; the Universe is not the same now as it was in the past.
And importantly, given the possibility that the conditions for intelligence to emerge are now in place, we shouldn't give up hope about our chances of discovering extraterrestrial life.