What is life?
This post will be a short and sweet answer to a question asked by one of my friends: What is the difference between plants, animals, bacteria, and fungi?
One common way to divide these different forms of life is by whether or not they have a nucleus. A nucleus is a coffee filter-like bag that surrounds the long strings of DNA inside a cell. It gives some separation between the code for cellular actions (the DNA) and the actions themselves. It provides some organization to the cell, so that many processes can happen all at the same time without getting too mixed up with each other.
Bacteria are different from plants, animals, and fungi: they are prokaryotes. This translates to 'before nucleus', as in bacteria are from a time before the nucleus evolved. Fungi, plants, and animals are all known as eukaryotes: this means that they all have a nucleus. These latter life forms are thought to have evolved later from bacteria, and from one common ancestor that was the first to have a nucleus.
There are many other small differences between these different types of life. Bacteria, fungi, and plants have cell walls, but animals do not. Fungi and bacteria include single-cell organisms: life forms that only consist of one cell. In contrast, the categories of plants and animals aren't usually considered to include single-cell organisms (organism is just another way to say living thing). And the list goes on. But, as you can already see, there are a lot of overlaps in the different categories. To an untrained eye, it can be hard to decide just by looking at something if it is a fungus or a plant, for instance. If you didn't know a mushroom was considered a fungus, wouldn't you think it had more in common with a flower than baker's yeast?
In past years, scientists did indeed categorize different types of life based on how they looked. But, as we learned more and more about life, the differences became too small and the boundaries too vague. That's why we now use a new system of classification to decide where things sit in the tree of life. What do we use? Ribosomes!
Let's back up a bit. Ribosomes are tiny machines made out of protein, and their function is to make more protein. Specifically, they translate messages from DNA (there are a few steps before this, but I'll leave it out for now) into protein. Ribosomes do this by linking together 'beads' of amino acids to make 'necklaces' of protein that then fold into all sorts of different 3D shapes. Ribosomes are completely essential to the functioning of any cell, because pretty much all of the machines in a cell are made of the same material: protein, which the ribosomes make!
A couple of pictures of ribosomes, courtesy of my boyfriend Robert Englmeier, the author of this paper!
Ok, so we have established that ribosomes are pretty cool. But why would we use these particular machines for detailed categorization? The reason stems from the fact that ribosomes are so essential to life. All known life has ribosomes, and because ribosomes need to function well for any life form to survive, they have relatively few mutations. An organism with a mutation in its ribosomes will only survive and pass on its ribosomes, mutation and all, to its children if the ribosomes are still functional. Together, this means two things: ribosomes mutate, or change, on a slow and evolutionary time scale, and they are everywhere in the tree of life.
Using slightly different sequences of biological molecules in ribosomes (think of these sequences as necklaces with beads in different orders again), scientists can divide organisms into different groups. They can also see which organisms evolved from which other organisms! Think of it like this: one 'necklace' has beads in the order red-red-yellow-blue-red, a second has the order red-red-yellow-green-red, and a third has the order red-red-purple-green red. By looking at the order of the beads, you can start to deduce an order of evolution: the third necklace was probably a variation on the second, which was a variation on the first. If you want to know more about this system and the cool dude who thought it up, google Carl Woese. He is pretty much the Einstein of biology.
In conclusion, what makes different groups of life different? The simplest answer, and the answer we use for categorization, is that they have tiny differences in their ribosomes. This is possibly a frustrating answer to the question. The take-home message for this week is, in some ways there are very few clear differences between different domains of life. Maybe this explains why scientists are often socially liberal: if there is so little difference between a human and a mouse, how can we see huge differences between two humans, regardless of their different backgrounds?
I'll leave you with that deep thought for this week.
Until next Monday,