Through many generations, gene variants caused by mutation, are transferred to descendants giving them new characteristics. With time, differences between organisms become more pronounced and we can follow their evolution by studying fossils, remnants of organisms and proofs of their activity preserved in stone. They show us individual steps of evolution and give us information about when each of the characteristics appeared throughout history. Earliest life forms were unicellular. Cells themselves were not found, but we do find signs of their activity. Multicellular life forms appear in Proterozoic. They were relatively large organisms without skeletons that were mobile, which we see from ichnofossils. In Cambrian period, organisms are getting more complex and there is a sudden appearance of myriad of organisms. All modern phylogenetic lines were probably formed then. During Ordovician, first vertebrates appear and we can find first plant fossils, mostly spores. Early representatives of modern plant groups appear in Devon, they develop complex conductive tissue, leaves and seeds and are growing to modern trees sizes, while vertebrates transition on land for the first time. Carboniferous and Permian organisms are subjected to high degrees of diversification and many modern groups appear, including synapsids, early reptile group from which mammals eventually evolved. Fossils of all modern animal groups appear during Mesozoik. Archaeopteryx, one of the most famous fossils ever discovered, appeared during that period and is a transitional form between reptiles and birds. Plants develop fruits and placental mammals are starting to appear, currently dominant group of organisms on Earth.