A recent review in Neuron (vol. 88 pp. 681 – 677 ’15) gives a possible new explanation of how our brains came to be so different from apes (if not our behavior of late).
You’ve all heard that our proteins are only 2% different than the chimp, so we are 98% chimpanzee. The facts are correct, the interpretation wrong. We are far more than the protein ‘bricks’ that make us up, and two current papers in Cell [ vol. 163 pp. 24 – 26, 66 – 83 ’15 ] essentially prove this.
This is like saying Monticello and Independence Hall are just the same because they’re both made out of bricks. One could chemically identify Monticello bricks as coming from the Virginia piedmont, and Independence Hall bricks coming from the red clay of New Jersey, but the real difference between the buildings is the plan.
It’s not the proteins, but where and when and how much of them are made. The control for this (plan if you will) lies outside the genes for the proteins themselves, in the rest of the genome (remember only 2% of the genome codes for the amino acids making up our 20,000 or so protein genes). The control elements have as much right to be called genes, as the parts of the genome coding for amino acids. Granted, it’s easier to study genes coding for proteins, because we’ve identified them and know so much about them. It’s like the drunk looking for his keys under the lamppost because that’s where the light is.
We are far more than the protein ‘bricks’ that make us up, and two current papers in Cell [ vol. 163 pp. 24 – 26, 66 – 83 ’15 ] essentially prove this.
All the molecular biology you need to understand what follows is in the following post — https://luysii.wordpress.com/2010/07/07/molecular-biology-survival-guide-for-chemists-i-dna-and-protein-coding-gene-structure.
The neuron paper is detailed and fascinating to a neurologist, but toward the end it begins to fry far bigger fish.
Until about 10 years ago, molecular biology was incredibly protein-centric. Consider the following terms — nonsense codon, noncoding DNA, junk DNA. All are pejorative and arose from the view that all the genome does is code for protein. Nonsense codon means one of the 3 termination codons, which tells the ribosome to stop making protein. Noncoding DNA means not coding for protein (with the implication that DNA not coding for protein isn’t coding for anything).
Well all that has changed. The ENCODE Consortium showed that well over half (and probably all) our DNA is transcribed into RNA — for details see https://en.wikipedia.org/wiki/ENCODE. This takes energy, and it is doubtful (to me at least) that organisms would waste this much energy if the products were not doing something useful.
I’ve discussed microRNAs elsewhere — for details please see — https://luysii.wordpress.com/2010/07/14/junk-dna-that-isnt-and-why-chemistry-isnt-enough/. They don’t code for protein either, but control how much of a given protein is made.
The Neuron paper concerns lncRNAs (long nonCoding RNAs). They don’t code for protein either and contain over 200 nucleotides. There are a lot of them (10,000 – 50,000 are known to be expressed in man. Amazingly 40% of them are expressed in the brain, and not just in adult life, but during embryonic development. Expression of some of them is restricted to specific brain areas. It is easier for an embryologist to tell what type a cell is during brain cortical development by looking at the lncRNAs expressed than by the proteins a given cell is making. The paper contains multiple examples of the lncRNAs controlling when and where a protein is made in the brain.
lncRNAs can contain multiple domains, each of which has a different affinity for a particular RNA (such as the mRNA for a protein), or DNA, or protein. In the nucleus they influence the DNA binding sites of transcription factors, RNA polymerase II, the polycomb repressor complex. The review goes on with many specific examples of lncRNA function — synaptic plasticity, neurotic extension.
Getting back to proteins, the vast majority are nearly the same in all mammals (this is where the 2% Chimpanzee argument comes from). Here is where it gets interesting. Roughly 1/3 of lncRNAs found in man are primate specific. This includes hundreds of lncRNAs found only in man. The paper gives evidence that hundreds of them have shown evidence of positive selection in humans.
So the paper provides yet another mechanism (with far more detail than I’ve been able to provide here) for why our brains are so much larger, and different in many ways than our nearest evolutionary ancestor, the chimpanzee. This is the largest molecular biological difference found so far for the human brain as opposed to every other brain. Fascinating stuff. Stay tuned. I think this is a watershed paper.