SCIENTISTS have found fossils that could rewrite the history of life in the rocks of northwest Scotland. These microfossils are the remains of cells with a complex internal structure. They suggest that these sophisticated microbes may have arisen in freshwater lakes rather than in the sea. Until now the earliest known examples of these complex-celled 'eukaryotes - a key step on the path to multi-celled plants and animals - from such terrestrial environments were from hundreds of million years later. The findings, published in Nature, show that complex-celled life was living in freshwater much earlier than scientists had previously thought. They are the first evidence for these microbes living in such environments before the start of the Cambrian era, some 450 million years ago. The grey shales where the microfossils were found are part of a sequence of rocks known as the Torridonian. They were laid down at the bottom of lake beds between 1 billion and 1.2 billion years ago, and now form much of the coast of northwest Scotland. The countrys unique geological history means theyve hardly changed since. 'The earliest generally accepted microfossils are in Canadian rocks that formed about 2 billion years ago in ocean environments, and people have generally assumed that the earliest complex microbes evolved later on in the sea, not in lakes, says Leila Battison, a PhD student at the University of Oxford and one of the studys authors. 'But marine sediments are preserved as rocks much more often than freshwater sediments, so until now theres been very little evidence of microfossils in lake sediments, she adds. 'Weve now found this evidence - its wonderful to slice open a phosphate nodule and find this incredibly diversity of early living things inside. One of the earliest and most vital stages in the development of life was the emergence of eukaryotes - organisms that have complex cells with inner structures such as nuclei and internal organelles like mitochondria. Before this, life was confined to the much simpler 'prokaryotes. Scientists have known since the late nineteenth century that Torridonian rocks were rich in microfossils. But initially theyd been classified as simple prokaryotes and forgotten about. Researchers revisited them in the 1960s and 1980s, but this is the first time s have recognised the strange and sophisticated features that some possess - features including spikes and even arm-like appendages. The researchers, from Boston College in the USA and the Universities of Oxford and Sheffield, re-examined samples of Torridonian rock taken from 17 different places across the area. 11 of these samples turned out to contain reasonable numbers of microfossils. Planet Earth