Biological kinds undergo a variety of changes during their life span, and these changes vary in degree by organism. Understanding that an organism, such as a caterpillar, maintains category identity over its life span despite dramatic changes is a key concept in biological reasoning. At present, we know little about the developmental trajectory of children's understanding of dramatic life-cycle changes and how this might relate to their understanding of evolution. We suggest that this understanding is a key precursor to later understanding of evolutionary change. Two studies examined the impact of age and knowledge on children's biological reasoning about living kinds that undergo a range of natural life-span changes--from subtle to dramatic. The participants, who were 3, 4, and 7 years old, were shown paired pictures of juvenile and adult animals and asked to endorse biological or nonbiological causal mechanisms to account for life-span change. Additionally, reasoning of 3- and 4-year-old participants was compared before and after exposure to caterpillars transforming into butterflies. The results are framed in terms of a developmental trajectory in essentialist reasoning, a cognitive bias that has been associated with difficulties in understanding and accepting evolution. (Contains 1 table, 3 figures, and 1 footnote.)