I don’t mean in the flesh; Luca has been dead and gone for a long time. But have you heard of him? An important individual was Luca. I am descended from him, and so are you. And everyone else, and everything else that is alive today. Luca is not really a him. Luca is really LUCA, which stands for Last Universal Common Ancestor. Luca lived about three and half billion years ago, and although we don’t know everything about it, we do know a lot. By definition, Luca was the organism from which all current living organisms are descended. There would have been other kinds of life around, but they didn’t last. Only the descendants of LUCA are on earth today.
We know a lot about LUCA because there are many characteristics that all current life forms share, and these are therefore characteristics that LUCA must have had.
LUCA was a small single-celled organism, with a plasma membrane. It had a cytoplasm with many proteins, each composed of either 16 or 20 amino acids arranged in a specific order as a linear polymer. The proteins made up all the structures of the cell, and, as enzymes, catalyzed all the required chemical reactions that allow life to exist. All of these proteins were produced within LUCA by a complex process that all cells today still share. The process starts with a double strand of DNA, which contains a sequence of bases arranged in a manner that codes for each of the proteins. Every group of three bases (a codon) stands for a particular amino acid. As is still true today, there were four kinds of bases, and so there were (and are) 64 possible codons, which means that some amino acids can be coded for by two, three, four or even six different codons. The specific genetic code, which is the identification of which codons code for which amino acids, is the same for you and me, oak trees, elephants, most bacteria, and LUCA[i]. (But we are all different because our DNA sequences are different, much as this blog post is different from the last one because of the sequence of the specific words, even though it’s written in the same language.
In order for this sequence to be translated into the correct proteins, LUCA used an incredibly complex molecular process that (with the rare exceptions mentioned) has not changed in any form of life for three and a half billion years.
The coded sequence is copied from one part of the DNA (called a gene) into a long stretch of RNA (the messenger RNA, or mRNA), which then interacts with another form of RNA called a ribosome. In the ribosome each codon of the RNA (which is the same as the codon on the DNA) is “read” one at a time. The reading process is both chemical and mechanical, and quite fascinating.
Since amino acids and nucleotide bases do not interact chemically, two different adapter molecules are required. One of these is a form of RNA called transfer RNA (tRNA), and there are different specific molecules of this kind of RNA for each amino acid.
There is also a group of enzymes called amino acyl tRNA synthetase (aaRS); again, different ones for each amino acid. The aaRS enzymes have binding sites for one and only one amino acid, and the same enzyme molecule has another binding site specific for the unique shape of the tRNA that has the anticodon for that amino acid. The enzyme then does the actual translation of the nucleotide-based code into the correct protein structure by joining the amino acid to the proper tRNA. The entire process has the technical name translation. Once bound together, the happy couple of tRNA and amino acid goes off to the ribosome, where the tRNA binds to the mRNA, and its amino acid is added to a growing chain making a new protein. This extremely ingenious and complex machinery operates in all living cells, and was present in LUCA as well.
We don’t know how LUCA got to be able to do this, or what happened before LUCA. In this sense, LUCA is like the big bang is for physics. Of course, we know that time existed before LUCA, and probably life did also, but what kind of life was it? And we have no idea how that life was able to produce a genetic code, and the mechanism for protein production that uses the code.
In fact, the same kind of mystery applies to all of the biochemical machinery found in all living cells. These include the mechanical wheels that produce the energy molecule ATP from sunlight or food, the membrane machines that selectively move molecules in and out of cells, the error correction systems, and a host of metabolic pathways by which all cells make the material they need.
Will we discover the answers to these mysterious origins some day? I believe we will, but not until we begin to explore new ways of thinking about biology, and the still unknown laws that control the truly miraculous processes of life.
[i] The rare, slightly different variant code that occurs in a few species of bacteria and protozoa evolved from the original LUCA code as well.)