A 'nearly pristine' 600 million-year-old sponge has been uncovered in southern China.  Scientists believe the sponge – which represents the most primitive animal on Earth - is the oldest ever found.  Its discovery pushes back the start of the evolution of multicellular animals to before the Cambrian explosion 541 million years ago.  Researchers want to pin down the date because they believe it is key to understanding early animal evolution.  The split between sponges and most other animals was a major event in the early history of life on our planet.  The sponge, found in the Doushantuo Formation, is just over one millimetre in height and width, according to researchers from the Nanjing Institute of Geology and Palaeontology.  To get a good look, scientists used scanning electron microscopy and X-ray technology.  The team has named it Eocyathispongia qiania and say that it is made up of hundreds of thousands of cells.