Animal life is a recent addition to Earth, relatively speaking. The planet formed about 4.5 billion years ago, with microbial life likely emerging between 4.3 and 3.7 billion years ago. It was not until several billion years later that animals started to rock up in a humble and vaguely familiar form: sponges.<\/p>\n
In a new study, scientists look closely into \u201cchemical fossils\u201d in rocks, indicating that sponges were alive more than 541 million years ago. While older evidence<\/a> of animal life is out there, their findings suggest ancient sponges emerged much earlier than most multicellular organisms and were possibly one of Earth\u2019s first animals.<\/p>\n The organic compounds in question are special types of steranes, known as 30-carbon (C30) sterols. They are a geologically stable form of sterols, the group of steroids that includes cholesterol, and are found in the cell membranes of complex organisms.\u00a0<\/p>\n The researchers were able to link these special steranes to a class of sea sponges known as demosponges, strongly suggesting that the \u201cchemical fossils\u201d are traces of early animal life.<\/p>\n \u201cWe don\u2019t know exactly what these organisms would have looked like back then, but they absolutely would have lived in the ocean, they would have been soft-bodied, and we presume they didn\u2019t have a silica skeleton,\u201d Roger Summons, study author and the Schlumberger Professor of Geobiology Emeritus in MIT\u2019s Department of Earth, Atmospheric and Planetary Sciences, said in a statement<\/a>.<\/p>\n Sponges<\/a> are still around today and, despite appearances, they are animals, not plants or fungi. This is because they are made of multiple types of eukaryotic cells that perform different functions, lack cell walls, and can feed, reproduce, and respond to their environment \u2013 key characteristics that separate animals from plants, fungi, bacteria, and other life forms.\u00a0<\/p>\n They are, however, simple animals, lacking complex organs, tissues, and specialized systems like nervous, digestive, or circulatory systems. They\u2019re essentially a soft, squishy blob of animal cells \u2013 but animal cells, nonetheless.<\/p>\n A short time after the sponges first appeared, planet Earth witnessed the unprecedented blossoming of many complex species. Known as the Cambrian explosion<\/a>, it marks a time when many major animal groups suddenly appeared in the fossil record. Before this, most life consisted of unremarkable blobs and simple structures, but afterward, ecosystems teemed with diverse and complex organisms.<\/p>\n