It may well be for all the complexity of the feather and the limb, the human head and its contents are the crowning success of evolution. The human head is the most complex part of our body. The three pound brain uses twenty percent of all of the energy of the body. The head has the brain, eyes, ears, nose, and tongue–our primary sense organs. The bones of the skull are intricate and complex with protective cranium, articulated jaws for biting, and tiny ossicles that make hearing possible. The neural connections are beyond counting—in the trillions. And throughout the animal kingdom, there are thousands of variations of complex heads. Think of hammerhead sharks, anteaters, toucans, turtles, and chameleons. Did the human head appear one day complete in all of its detail, or did it appear as an evolutionary process—which, perhaps—is still ongoing?
Two simple sea creatures, the larvacean and the sea squirt, are the closest living relatives of the vertebrates, and they have no heads at all. However, they do have the genes to make a head. The larvacean is a tiny gelatinous tadpole which lives in a floating house made of its own mucous. Its rudimentary nervous system is organized along a simple nerve cord that runs along its back. The related sea squirt starts life as a swimming larva with a rodlike stiffener in its tail. When it reaches maturity, it drives its front end into the ocean floor, eats most of its nervous system, and turns it body into a basket for filtering food particles. The potential for a vertebrate head in each of these simple creatures is found in the front tip of larvaceans and larval sea squirts. They each have a small brain-like organ with 360 nerve cells where vertebrates have heads. The pattern of development of that small organ is strikingly familiar to what is seen in vertebrates, and some of the same genes that build human brains are present there. The genes are located front, middle, and rear as ours are.
Sea squirts have been found to have primitive cousins of neural crest cells, the cells which build much of the head in developing vertebrate embryos. The sea squirt neural crest cells emerge along the back of the developing embryo and migrate through the body. Unlike vertebrates where such cells finally produce a skull, the sea squirt’s cells become pigment cells which make the creature’s brilliant bodies. It is postulated that around 500 million years ago our ancestors may have resembled these modest little beings, already equipped with genes and cells that would later, by scores upon scores of mutations, sculpt into the faces and brains of humans and other vertebrates. More attention to humans, and especially the brain and head in future blogspots in this journey of understanding and appreciation of biological evolution.
The plant world operates in much the same way genetically as does the animal world. Darwin expressed his confusion about flowers which were, for him, “an abominable mystery”. Flowers appear to have emerged in the fossil record quite suddenly, and quickly presented an exquisite complexity. Modern research reveals that the first flowers almost undoubtedly split off from their closest living relatives, the gymnosperms (which include pines and other conifers, cycads, and ginkgoes), which produce seeds but not flowers.
Before a flower takes shape, sets of genes mark out an invisible map at the tip of the stem, a map almost identical to that found in animal embryos. The genes divide the tip into concentric rings. Guided by the genes, cells in each ring develop into different flower parts—sepals in the outer ring and sex organs in the innermost rings. The genes that build flowers are older than the flowers themselves. Gymnosperms, some 800 kinds of flowerless plants, carry flower-building genes even though they do not make flowers. Although the genes are present, their function is unknown in the gymnosperms. Flowers came into being by a chance duplication of a gene, freeing one of the genes to proceed on in its evolutionary progress to become a flower. Flowers became more complex, and some of their parts gained new functions such as luring insects by the same extremely long process of having a great many mutations until the 250,000 flowering plants of today came into existence. All of these extraordinary changes came about over eons of time and millions of generations and gene mutations—the evolution of living things by natural selection as demanded by the environment and facilitated by the natural functions of genes, fundamental structures of all living beings, flora and fauna. As ignominious as it may seem, humans share a substantial of their genes with lettuce. continued…