with millions of different species on earth we need some way to classify them our ancestors often did it based on what a species looked like for example a black bear is given to a brown bear which is different to a polar bear but it's hard to come up with names like this for everything and different people around the world had different names for the same thing like the grizzly bear which is just a type of ground bear the other problem is that simple names don't tell us much about how related the different species are for example grizzly bears koala bears and panda bears aren't really related at all to try and fix all these problems in the 1700s a man named carl linnaeus proposed a new type of classification which groups species together according to their characteristics and bone structures we call this the linnaean system and it groups species into kingdoms such as plant or animal then phylum class order family genus and finally species to make the system usable by all different countries and because it was designed hundreds of years ago the names are mostly in latin for example humans are animalia chordata mammalia primates ominoday homo and sapiens now this is obviously too much to say every time that you want to describe a species but that last category of species often isn't enough as multiple species could have the same one just like how lots of people are called tom so linnaeus instead proposed the binomial naming system where we name species by their genus and species names for example humans would be homo sapiens as homo is our genus name and sapiens is our species name and whenever you write a name like this you should do it all in italic and only capitalize the first letter of the genus name like we've shown here this system works pretty well and we still use it today however as our microscopes improved and we learned more about the internal structures of different species new models of classification have been proposed and although they didn't replace the linnaean system they kind of got combined with it for example in the 1990s a different cul this time carl woes proposed the three domain system using evidence that he had gathered from new techniques that could analyze rna sequences he found that some species were less related than we thought and so he introduced three new categories called domains that were placed above kingdoms one of these is eukaryota which contains all the organisms that have eukaryotic cells similar to ours such as plants fungi protists and other animals meanwhile bacteria refers to the very tiny single-celled prokaryotic organisms that are found almost everywhere and finally archaea which were originally thought to be bacteria are actually a different type of prokaryotic cell which are often found in extreme conditions like hot springs and salt lakes you might sometimes hear bacteria being referred to as true bacteria and archaea being referred to as primitive bacteria the last thing we need to look at are evolutionary trees these show us the evolutionary relationships between different species or groups by linking common ancestors for example we can see here that birds are more closely related to tyrannosaurus rex's than they are to bats because t-rexes and birds share a more recent common ancestor just here and we can also see that bats and birds are more closely related to each other than either are the spiders because again they have this recent common ancestor whereas their common ancestors spiders is all the way back here which means it's further back in the past different evolutionary trees can look quite different this here is the same information but just in a different form the main thing to remember is that whenever a line splits in two it's because a common ancestor is splitting into two different species in order to learn all these relationships in the first place scientists had to compare the structure and dna of different species looking not only at living species that are around today but also the fossil records when it comes to remembering the order of all these groups the best strategy is to find a good mnemonic which is just a phrase you use to help you remember terms in a list for example if you remember dear kate please come over for great spaghetti then you'll be able to work out the order of domain kingdom phylum and so on because the first letters of each word are the same and of course feel free to change around a couple of the words to make it easier to remember often the voodoo ones are the best and that's how we classify organisms if you found it useful then please do give us a like and we'll see you next time