The correct answer is birth.
At birth, females infants on an average appear to have a more developed central nervous systems compared to males. This is apparent because female infants shower higher levels of responsiveness to stimuli such as sounds, touch and faces. Additionally, female infants typically acquire linguistic abilities and vocalize earlier than male infants.
U·ni·cel·lu·laradjective
BIOLOGY(of protozoans, certain algae and spores, etc.) consisting of a single cell.(of an evolutionary or developmental stage) characterized by the formation or presence of a single cell or cells.
Hope this helps!
Fossilization so rare because it takes such a long time.
