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 different 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 brown bear The other problem is that simple names don't tell us much about how related the different species are For example grizzly bears and koala bears aren't really related at all To try and fix all these problems in the 1700s a man named Carl Lanaeus proposed a new type of classification which group species together according to their characteristics and bone structures We call this the Lan system and it groups species into kingdoms such as plant or animal then film class order family genus and finally species To make this 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 anamelia cordata mamalia primates amen 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 Lanaeus 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 done 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 Lenan system they kind of got combined with it For example in the 1990s a different Carl this time Carl Wos 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 ukareota which contains all the organisms that have ukarotic cells similar to ours such as plants fungi protests and other animals Meanwhile bacteria refers to the very tiny single-sellled proarotic organisms that are found almost everywhere And finally archa which were originally thought to be bacteria are actually a different type of proarotic 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 archa being referred to as primitive bacteria Each of these domains is then subdivided into kingdoms and then in turn all of those smaller groups proposed by the lenan system that we just went through 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 Rexes 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 to spiders because again they have this recent common ancestor Whereas their common ancestor with spiders is all the way back here which means it's further 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 of 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 at the fossil records When it comes to remembering the order of all these groups the best strategy is to find a good pneummonic 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 film 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 If you haven't heard yet you can find all of our videos on our website cognito.org You'll also find questions flashcards exam style questions and pass papers And we track all of your progress so that you always know what to study next So sign up for free by clicking here or browse our playlist here on YouTube