Peter Ward, a paleontologist, and Donald Brownlee, an astronomer, co-authors of the 2000 book Rare Earth[1].
The engine uses them for the idea that simple life may be common but complex life is extremely rare.
In Rare Earth they argued that basic microbial life could be widespread across the universe, but complex multicellular life requires an unlikely stacking of rare planetary conditions all coming together.
Their thesis frames Earth's advanced life as a statistical anomaly, a key reference point in debates about how alone we might be.