Final answer:
The lowest taxonomic level shared by humans and spiders is the kingdom level, where both are categorized under the kingdom Animalia. They share this classification as multicellular, heterotrophic organisms that can move.
Step-by-step explanation:
The lowest taxonomic level shared by both humans and spiders is the kingdom level. Humans belong to the kingdom Animalia, class Mammalia, order Primates, genus Homo, and species sapiens. Spiders fall into the kingdom Animalia, class Arachnida. After the kingdom level, their classifications diverge significantly, reflecting their vastly different evolutionary paths.
The hierarchical system of taxonomy, known as the Linnaean system, begins with the broadest category, domain, and progresses to increasingly specific categories, ending with species. Within this system, humans and spiders share the same kingdom but then separate into different classes, orders, and further down the hierarchy. This common kingdom indicates that both are multicellular, heterotrophic organisms that are capable of movement at some stage in their life cycle, traits they share with many other forms of life.