All living organisms on earth are related. All of them. Every bacteria, tree, bird, snake, cat, dog, human. There is overwhelming evidence that we all descend from a single common ancestor.
LUA, or the last universal ancestor, lived between 3.5 and 3.8 billion years ago and, as far as we know, all life on earth descends from it. It was probably a very simple cell organism, with a cell membrane and a DNA ring. Side by side, it would not have looked much different to a current-day bacteria.
That’s not to say that life did not start and stop several times. The LUA and its descendants, which includes all life as we know it, were the lineage that stuck around. There could have been hundreds of lifeforms that had independently sprung up before it, but we simply can’t know.
It is incredibly unlikely that we will ever find a complex organism on earth that has evolved from a different ancestor. A study in 2010 by Walter Theobald of Brandeis University compared 23 proteins from 12 different species. Theobald took these proteins and, with the help of a computer simulation, determined that having a single common ancestor is 102860 times more likely than having multiple ones. That’s not a typo. That’s a 1 with over 2000 zeroes after it. To put it in perspective, there are 1080 atoms in the universe (according to the latest estimates).
When Theobald included the possibility of gene swapping between species, having a common ancestor was 103489 times more likely than multiple ones. Which is also, excuse my non-scientific language, a giant, enourmous monster of a number.
Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.
-Charles Darwin, On the Origin of Species
Though plants, animals, fungi and so on, likely share this ancestor, multicellular life has independently evolved dozens of times throughout the history of our planet. Once for animals, once for green algae and plants, probably several times for fungi, slimes, etc.
There are advantages and disadvantages to multicellularity. Multicellularity allows division of labour, cells can become specialised, form tissues and become excellent at specific tasks. On the other hand, multicellular organisms also suffer from cancer, which is what happen when a cell cannot control its growth anymore.
Becoming multicellular opens the gateway for all sorts of remarkable innovations that would be impossible for single cells. By being larger and by being made up of numerous cells, organisms can have a division of labor and also respond to their environment in new and sensitive ways, all adaptations that have led to their success.
-John Tyler Bonner, The Origins of Multicellularity
And of course, multicellular organisms eventually developed consciousness. Which means that you can read this information and understand it from the other side of the world thanks to the technological advancements that humanity has made. Sure, bacteria can live anywhere, but it takes a multicellular organism like a human to bend nature to its will. Or to start to question its own existence.
For more information of this topic, the huge “Evidence of Common Descent” article on wikipedia is a great place to start.
-Francesco (follow me on twitter)
References and further reading
- John Tyler Bonner – The Origins of Multicellularity – Integrative Biology
- Theobald DLI (2010) – “A formal test of the theory of universal common ancestry” – Nature
- All Modern Life on Earth Derived from Common Ancestor – Discovery News
- Doolittle WF (2000) -”Uprooting the tree of life” – Scientific American