<Tr> <Td> Isoleucine * </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Leucine * </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Ethionine </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr>

Which was least likely to have been a component of earth's atmosphere before life began