<Li> Response to external and internal stimuli such as changes in temperature, pH or nutrient levels . </Li> <Li> Cell contents are contained within a cell surface membrane that contains proteins and a lipid bilayer . </Li> <P> The last universal common ancestor (LUCA) is the most recent organism from which all organisms now living on Earth descend . Thus it is the most recent common ancestor of all current life on Earth . The LUCA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era). The earliest evidence for life on Earth is graphite found to be biogenic in 3.7 billion - year - old metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion - year - old sandstone discovered in Western Australia . Although more than 99 percent of all species that ever lived on the planet are estimated to be extinct, there are currently 10--14 million species of life on Earth . </P> <P> Information about the early development of life includes input from many different fields, including geology and planetary science . These sciences provide information about the history of the Earth and the changes produced by life . However, a great deal of information about the early Earth has been destroyed by geological processes over the course of time . </P>

What are most organisms on earth classified as