 Researchers have identified traces of what they believe is the earliest known prehistoric ancestor of humans – a microscopic, bag-like sea creature, which lived about 540 million years ago. Named Saccorhytus, after the sack-like features created by its elliptical body and large mouth, the species is new to science and was identified from microfossils found in China. The research team says that Saccorhytus is the most primitive example of a category of animals called "deuterostomes" which are common ancestors of a broad range of species, including vertebrates (backboned animals). If the conclusions of the study, published in the journal Nature, are correct, then Saccorhytus was the common ancestor of a huge range of species, and the earliest step yet discovered on the evolutionary path that eventually led to humans, hundreds of millions of years later. At first glance, however, S. coronaries does not appear to have much in common with modern humans. The actual creature was probably no more than a millimetre in size 
 
 Researchers have discovered the earliest known ancestor of humans - along with a vast range of other species. The study was carried out by an international team of academics, including researchers from the University of Cambridge in the UK and Northwest University in Xi’an China, with support from other colleagues at institutions in China and Germany. [7 Theories on the Origin of Life] 
 
 "We think that as an early deuterostome, this may represent the primitive beginnings of a very diverse range of species, including ourselves," Simon Conway Morris, a professor of evolutionary palaeobiology at the University of Cambridge, said in a statement. He told BBC News: "To the naked eye, the fossils we studied look like tiny black grains, but under the microscope the level of detail was jaw-dropping. All deuterostomes had a common ancestor, and we think that is what we are looking at here.” 
 
 Degan Shu, from Northwest University, added: “Our team has notched up some important discoveries in the past, including the earliest fish and a remarkable variety of other early deuterostomes. Saccorhytus now gives us remarkable insights into the very first stages of the evolution of a group that led to the fish, and ultimately, to us.” 
 
 Most other early deuterostome groups are from about 510 to 520 million years ago, when they had already begun to diversify into not just the vertebrates, but the sea squirts, echinoderms (animals such as starfish and sea urchins) and hemichordates (a group including things like acorn worms). However, these incredibly diverse animals made it hard for scientists to figure out what the common deuterostome ancestor would have looked like, the researchers said. The Saccorhytus microfossils were found in Shaanxi Province, in central China, and pre-date all other known deuterostomes. By isolating the fossils from the surrounding rock, and then studying them both under an electron microscope and using a CT scan, the team were able to build up a picture of how Saccorhytus might have looked and lived. Dr Jian Han, of Northwest University, said: “We had to process enormous volumes of limestone – about three tonnes – to get to the fossils, but a steady stream of new finds allowed us to tackle some key questions: was this a very early echinoderm, or something even more primitive? The latter now seems to be the correct answer.” 
 
 In the early Cambrian period, the region would have been a shallow sea. It was also covered with a thin, flexible skin, suggesting it had muscles of some kind that could perhaps help it wriggle around in the water and engulf food with its large mouth, the researchers said. Saccorhytus was also covered with a thin, relatively flexible skin and muscles, leading the researchers to conclude that it moved by contracting its muscles and got around by wriggling. Saccorhytus had a large mouth, relative to the rest of its body, and probably ate by engulfing food particles, or even other creatures. Small, conical structures encircling its mouth may have allowed water it swallowed to escape from its body. These, the scientists suggest, might have allowed the water that it swallowed to escape and so might have been a very early version of gills. It probably moved around by wriggling 
 
 The researchers were unable to find any evidence that the animal had an anus, which suggests that it consumed food and excreted from the same orifice. [See Images of the Bag-Like Animal & Other Cambrian Creatures] 
 
 "If that was the case, then any waste material would simply have been taken out back through the mouth, which from our perspective sounds rather unappealing," Conway Morris said. Technically, it is possible to estimate roughly when species diverged by looking at differences in their genetic information. Unfortunately, before a point corresponding roughly to the time at which Saccorhytus was wriggling in the mud, there are scarcely any fossils available to match the molecular clock’s predictions. Some researchers have theorised that this is because before a certain point, many of the creatures they are searching for were simply too small to leave much of a fossil record. These creatures, called loriciferans, measured about 0.01 inches (0.3 mm) and, like S. coronaries, lived between grains of sand, the researchers said in a study published online today in the journal Nature Ecology and Evolution. Credit: S Conway Morris/Jian Han 
 
 Now that researchers know that deuterostomes existed 540 million years ago, they can try to match the timing to estimates from biomolecular data, known as the "molecular clock."
– Our very primitive beginnings look to have been very primitive indeed. It turns out the earliest known ancestor of humans was a sea creature a millimeter in size that likely lacked an anus. In a study published in Nature on Monday, scientists named Saccorhytus coronaries as a 540 million-year-old member—at that age the oldest one—of a category of animals called deuterostomes. The BBC explains deuterostomes gave rise to vertebrates. "All deuterostomes had a common ancestor, and we think that is what we are looking at here," researcher Simon Conway Morris tells the BBC. As for what they're looking at, "the bag-like body bears a prominent mouth and associated folds," the researchers write. Live Science reports the deuterostome groups scientists have previously discovered were at most 520 million years old, and had begun evolving into vertebrates, echinoderms (ie, starfish), and hemichordates (acorn worms). That left scientists stumped as to what a common ancestor would have looked like. Now they say they know. "We had to process enormous volumes of limestone—about [3 tons]—to get to the fossils, but a steady stream of new finds allowed us to" answer that question, says co-author Jian Han of the find, made in China. Two other details, from a Cambridge press release: The minuscule creature is thought to have lived between grains of sand on the sea bed, and it featured small conical structures that the water it ingested may have exited through "and so were perhaps the evolutionary precursor of the gills we now see in fish." (Scientists have found Earth's oldest civilization.)