This value remains particularly controversial. Those who favor a low value, such as the biologist Ernst Mayr, point out that of the billions of species that have existed on Earth, only one has become intelligent and from this, infer a tiny value for f i. Likewise, the Rare Earth hypothesis, notwithstanding their low value for n e above, also think a low value for f i dominates the analysis. Those who favor higher values note the generally increasing complexity of life over time, concluding that the appearance of intelligence is almost inevitable, implying an f i approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable. (See Criticism).