The kind of plants and animals we see on our planet today have not been there since it was formed billions of years ago. This is obvious from the many bones and fossils that have, and are still being discovered in different parts of the world. They all indicate that different creatures roamed the Earth at different periods. But, have they left something behind in the form of some property shown by an animal or plant, even today? Even more importantly, are these ancient creatures a connecting link between humans and other modern animals? The answer to these questions lies in a concept called 'synapomorphy', which is explained below with some of its examples. A synapomorphy is a common property shown by two or more groups of organisms, that can be traced back to the most recent ancestor that both the groups evolved from. However, this character may not be shown by other closely-related groups, because some of them may have evolved further, or lost the character altogether. Synapomorphic traits are those which have appeared only in the last common ancestor for the first time, and not in more primitive organisms. This helps researchers find out which ancestral organism first evolved a particular character, that is now seen in different species or populations. It helps them establish evolutionary relationships between different groups of organisms, like birds, reptiles, and mammals, that show similar properties. Synapomorphies play an important role in the system of 'cladistics', which groups organisms into different categories, or 'clades', depending on their common ancestors. Only synapomorphic traits can be used to link various groups together, because if a property shared by different organisms is not ancient, it means that they share a recent, common ancestor who developed it. The category of 'apes', which includes humans, are distinguished from other primates by flatter rib cages, more mobile shoulder joints (for swinging on branches), grinding teeth with 4 raised bumps, and the lack of a tail. However, other mammals, and even primates like monkeys, do not show these traits, indicating that apes have derived them from a recent common ancestor. Humans and cats, despite being from different groups (primates and carnivores respectively), show similarity in their forearm bone structure. Though they evolved by different paths, these common traits were passed down to both groups by the last common ancestor that they shared. Similarly, the bones in bat wings have a similar arrangement to human arm bones, with the bones at the top being very similar to those of our hands. The category of 'land plants' includes different groups, like the Coleochaete (a type of algae), liverworts (flowerless plants), conifers, and angiosperms (flowering plants). Despite this, only liverworts, conifers, and angiosperms show a multicellular sporophyte (spore-producing stage), while the Coleochaete does not. This is a synapomorphy passed down to them by the last ancestor that they shared. The genus Homo includes all great apes, such as Homo erectus (upright man), Homo neanderthalensis (neanderthals), and Homo sapiens (modern man). Despite the differences between them, they all shared the similar property of having large brain-cases, indicating their higher intelligence. This is because, their most recent and common ancestor―Australopithecus―evolved this trait for the first time, and passed it down to them. Of these species, we are the only ones still surviving today. The superclass Tetrapoda includes all four-footed animals, such as reptiles (like lizards and crocodiles), amphibians (like salamanders and frogs), birds, and mammals. Despite their enormous diversity, all these animals show four limbs and an amniotic egg (embryo development inside an egg), which indicates that they evolved from a common ancestor. Interestingly, some synapomorphies are seen only in mammals, such as hair and mammary glands, and are absent in other tetrapods. Birds belong to the class Aves, and crocodiles, snakes, and lizards belong to Reptilia. One synapomorphy which indicates that they share a common ancestor is their tendency to lay eggs with shells.