This article discusses the organisms that live in the space between the sand grains, which is referred to as the interstitium. Various metazoan taxa inhabit this area, including nematodes, copepods, and annelids. The main objective is to discuss the varying hypotheses that explain the origins of interstitial animals. The hypotheses include three scenarios: ancestral bilaterians were first to inhabit this area, the process of progenesis allowed for organisms to adapt to living in the interstitium, or that the body size of organisms would slowly decrease as time went on and lead to organisms coming to inhabit the area. The researchers provided an approach based on phylogenomics to create a likely phylogenetic tree of progenesis. The idea of an “Archiannelida” is rejected as a possible hypothesis. Hypotheses including progenesis are determined to be applicable, as the forefront feature of organisms that live in this environment is small body size. Adaptation over time for smaller body size is a key aspect pertaining to progenesis. Another imperative factor to consider is miniaturization, when looking into the potential history of these interstitial organisms.
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