It is highly unlikely that the last universal common ancestor was the first organism to exist. Now, we dont know what created life, but at this point, the most likely hypothesis, in my opinion, is Abiogenesis. This creation may not have even happened on earth. It could have occurred on Venus or Mars and was transported to earth via Panspermia.

Regardless of where life born, it was not by a single nucleotide. There was an estuary of nucleotides, the 'primordial soup'. An entire population of nucleotides was requred so that: at least 2 nucleotides, of the entire population, could bond, and then randomly happen to collide, such that they did (bond).

Currently there are 5 nucleotides: Adenine, Guanine, Cytosine, Uracil and Thymine. Uracil and Thymine are interchangeable (not exactly, but stick with me). So all of lifes' genetic code can be transcribed in sequences of A, T, G and C. A bonds to T and C to G. A cannot bind to A. Neither any other single nulceotide Therefore, you need at least 2 nucleotides before any of them can self replicate.

I was taught in one of my biology classes that some organisms, specifically starfish, have different nucleotides than the rest of life. Unfortunately, i have to succeed finding a source.

Anyway, lets get back to the idea that began this discussion, common ancestors. There were Several. Animals and fungi share an ancestor with an archaea and a bacteria, while plants share it with the aforementioned archaea and bacteria in addition to a 2nd bacteria. The archaean common ancestor was a large (microscopically speaking) low energy slow moving cell. Imagine a car without an engine or finished interior. The bacterial common ancestor to animals, fungi and plants was a small, high energy fast moving cell, which eventually came to be the mitochondrion. Imagine and engine. Now, imagine the previously mentioned bare bones car, lifting it's hood and engulfing an engine. That cell was the most recent common ancestor of plants, animals and fungi. Later, by the same method, plants went on to engulf a the 2nd bacteria which became chloroplasts. The process of engulfing bacteria is known as phagocytosis. To help you image what phagocytosis looks like, here is a video of a white blood cell phagocytizing a bacteria.