In Ancient cultures, human believed that the heart is the center of all thoughts and emotions, however, nowadays neuroscience and biology science prove that nervous system is the power behind the emotion and human evolution. The human nervous system is a wonderful thing, as many complex elements and it’s consisted of many important things. During each human life span, the study of the development of nervous system gets more interesting. Therefore, due to technology of nowadays, neuroscientist be able to reveal the secret of the human nervous system evolution and development.
Human brains are composed of three central elements; Neurons, the cells that transmit the information from one place to another; Synapses, the connections between neurons and the circuit, the specific pattern of connections between the neurons. without all these elements, human brains will fail. Without synapses, human brain will fail to store memory. Without neutrons there would be no electrical activity and without a specific circuit no meaningful computation can be performed.
Another wonderful example of irreducible complexity. So how human brain is evolving? First came to neuron, a neuron is simply a cell that transmits an electrical impulse from one location to another.
The electrical impulse is simply a passive wave of ions. Once started, it will propagate through the cell on its own, just like ripples on the surface of a pond. All the cell needs are a means of starting such a wave, and a means of using the electrical wave to do something useful. In order to start an electrical wave an ion channel is needed.
Ion channels are proteins in the cell membrane that allow one or more types of ions to move in or out of the cell in response to a stimulus. DNA sequences reveal that the ion channels used in the neurons of animals had their origin in bacteria where they were used to regulate ion concentrations inside the cell. At the opposite end of our early neuron existed a special type of ion channel, it’s called a voltage-gated ion channel. These open in response to a passing electrical wave and can allow special ions like calcium to enter the cell which goes on to activate a multitude of cellular processes. Again, DNA sequencing reveals that voltage-gated ion channels also had their origins in bacteria. Therefore, the first neurons were simply co-op with bacterial ions channels put to the new task of transmitting information. Such basic electrical signaling evolved even before the appearance of multicellular animals. The single cell paramecium generates a voltage change when it bumps into obstacle. The wave of ions travels across the cell reversing the beating of its cilia, allowing it to change direction. Moving to multi cellular organisms, such proto neurons would have been highly useful even without synapses. An electrical wave can traverse macroscopic distances orders of magnitude faster than simple diffusion.
Therefore, as body size enlarged, the distance between locations where a stimulus may be sensed and where an action is required such a sensing a touch and contracting a muscle would also enlarge, thus, requiring a mean for rapidly transmitting information. Early multicellular animals through gene duplication, mutations and natural selection, co-opted preexisting ion channels originally evolved in bacteria to produce proto-neurons to accomplish just that. Over time the family of ion channel genes would have enlarged allowing for more specialized functions such as the active propagation of electrical waves known as action potentials. Action potentials involve specialized sodium and potassium ion channels, that for an electrical analogy act as repeaters boosting the signal as it travels and can transmit information faster and over longer distances than simple passive waves. Without which organism could never have achieved the size they did.
Next is synapses, early synapses are a simple pores between neighboring cells. Such pores can be formed by gap junction proteins which evolved around the time multicellular organisms first appeared, allowing molecules to diffuse between neighboring cells. These pores, called an electrical synapse, allow the electrical wave to travel from one cell to another and can still be found in the brains of almost all animals today. So, the first synapses were co-opted molecular pores put to the new Trask or permitting electrical waves to travel between cells. The other type of synapses, called a chemical synapse, is a bit more complex. Electrical wave in one neuron causes the cell to release a chemical called a neurotransmitter. The neurotransmitter binds to receptors on a nearby neuron. When bound those receptors open their ion channels starting a new electrical wave. Again, such a system appears irreducibly complex. Why release the neurotransmitter if there is no receptor for it to bind? Why have such a such a receptor if no cells are releasing the chemical. However, if look at the proteins in animal kingdom involve in making synapse, their origin becomes apparent.
The receptor came first, glutamate is the main excitatory neurotransmitter. Analysis of DNA sequences reveals that glutamate receptors existed before the divergence of plants and animals and therefore before the emergence of multicellular life. And the glutamate binding domain goes as far back as bacteria. Early, unicellular organisms would have used this receptor to sense glutamate, an important cellular metabolite, in their environment. Later multicellular organisms co-opted this protein by evolving cells to release glutamate near another cell already expressing glutamate receptors. And thus, the chemical synapse was born. Over time through gene duplications, mutation and natural selection, different types of neurotransmitters and receptors evolved. Some allow positively changed ions to flow into the cell leading to an electrical wave resulting in the release of neurotransmitter from that cell, and are thus term excitatory synapses, but others allow negatively charged ions to flow into the cell and are thus term excitatory synapses, but others allow negatively charged ions to flow into the cell. These can essentially cancel out any positive waves, thereby preventing the cell from releasing neurotransmitter, and are therefore called inhibitory synapses. But what was the use of synapses if complex circuits weren’t already genetically encoded? It was the origin of synapses that allow circuit patterns to begin evolving under natural selection. And as it displays, even extremely simple circuits can be useful to an organism.
The earliest circuits transduced sensory input to motor output in reflexive manner, basically input-output with little to no computation in the middle. An example are the nerve nets in sea anemones. More complex circuits can be completely genetically encoded such as the “brain” of the nematode C. elegans which only contains 302 neurons. But even simple 4 neuron circuits can make decisions, 1 neuron is enough to store short-term memory, and 1 synapse is enough to store the long-term memory. Leaning can occur through modification of the network or plasticity of the strength of synaptic connections. Each of these processes has co-opted pre-existing cellular machinery.