Humans are complex, no doubt about it. Are we more complex than a sperm whale? I would debate that (either side actually). I mean, I don’t know a single human that could dive deeper than most submarines, stay there for several hours, build a 3D map of the environment with sound, and then hunt. But I digress.
There are organisms with more genes than humans. There is a tiny little worm called Caenorhabditis elegans (image below) that is about one millimeter in length, yet has 20,000 genes. Humans have barely 40,000 (depending on who you ask).
How come humans are way more than twice as complex as this little nemotode?
The key appears to be not in the genes themselves, but how the genes are regulated. Humans have about 3,000 transcription factors, whereas the worm has barely a thousand.
A transcription factor is a protein that binds to a specific DNA sequence. This binding can change how that gene is transcribed into mRNA, hence the name ‘transcription factor’. As a quick reminder, a DNA sequence is transcribed into messenger RNA (mRNA), which then travels out of the nucleus of cell and into a ribosome. The ribosome translates the mRNA sequence into a protein. This is the central dogma of molecular biology. DNA -> mRNA -> protein.
These transcription factors can basically do two things: block transcription or promote transcription. They often act in concert with other proteins and may rapidly increase transcription, slow it to a crawl, or stop transcription entirely.
The transcription factors can respond to genetic or chemical signals that come from the organism or the environment. For example, the estrogen receptor transcription factor responds to estrogen secreted by the ovaries. The estrogen actually crosses the nuclear membrane and directly interacts with the estrogen receptor, which then (because of a shape change caused by the estrogen) can attach to a particular sequence of DNA and ‘turn on’ (cause transcription) of certain genes in the cell.
Hox transcription factors are vitally important in development and body organization of an organism. By injecting, say a fruit fly egg, with Hox transcription factors, scientists can cause a mandible to turn into a leg or cause eyes to grow on the knees of the legs or other ‘monster’ effects.
Human complexity is not based on the number of genes, but on their regulation. The number of genes is fairly constant in animals, but the transcription factors that control gene expression are the critical links and cause the massive developmental changes that produce our complexity.
For example, we can take a eye development gene from a mouse, put it into a fruit fly and the fruit fly will develop perfectly normal eyes. The gene for eye development is basically the same in almost every animal on the planet. But the transcription factors that control when and how the eye genes are turned on can result in compound eyes of the fruit fly, the pin-hole camera eyes of a chambered nautilus, the vertebrate backward eye with it’s blind spot, or a cephalopod eye that’s just like a vertebrate eye, but without the blind spot.
This also explains the development of different body plans. The vertebrate body plan that we humans have isn’t that different from the fish ancestors we came from. In fact, developmental studies have shown that the gill arches that appear in both mammal and fish embryos come from the same genes. However, different regulation results in fish getting gills and humans getting everything from the bones of the middle ear to the hyoid bones.
Neil Shubin’s Your Inner Fish describes a lot of these changes and does so very well.
