The first animals to inhabit the Earth may have been sea sponges, a new study by geochemists from the Massachusetts Institute of Technology (MIT) has suggested. In their work, the researchers linked so-called 'chemical fossils' found in ancient rocks to the ancestors of a class of modern-day sea sponges known as demosponges. These rare steroids were linked to ancient sponges, but the findings sparked debate, with some suggesting alternative origins, including non-biological processes. When they tested ancient rock samples, they found both C₃₀ and unexpectedly high levels of C₃₁ steranes, the fossilized forms of these sterols. Do you have a question about sea sponges?