Background: Life, Animals, and Nervous System Theory
Earth is filled with Life, Yes. But what exactly is "Life?" And what forms of Life have nervous systems? Here we give a brief primer on this active scientific arena.
What will you learn?
In this write-up you will read about the organization of living beings and the types of nervous systems therein.
- This is a purely theoretical writeup where we speak of the origins. No background is necessary
The big questions of biology are:
These three questions have challenged scientists and thinkers since civilization began, but we have made progress in answering questions one and two . Question three remains the unknown of unknowns .
We tackle question one here: what is Life? In the 1970's, Chilean scientists Humberto Maturana and Francisco Varela wrote of 4 properties common to all living systems in their influential philosophical work "Autopoiesis: On Machines and Living Things."
The life we observe on Earth is typically divided into three domains: the Bacteria, the Archaea, and the Eucaryota. Archaea and Bacteria are both single-celled life forms without nuclei, but the two groups have different metabolic chemistry. Bacteria we are familiar with in both their friendly (intestinal flora) and unfriendly forms (diseases). Archaea are famous for living in extreme environments - hot springs, acidic depths, extreme cold, etc, where not much else can live. Eucaryota (or eucaryotes) have nuclei, organelles like mitochondria and chloroplasts, and are themselves further classed into various groups such as the single-celled "protist" forms, the intermediate forms (single cells that can form colonies), and the true multicellular forms like us.
It is to these multicellular eukaryotic forms that we direct our attention. Broadly speaking, there are three types of multicellular life . First, we have the plants. Wonderful multicellular lifeforms that use the energy of the sun to grow and make their own food.
And finally, we have the animals. But what is an animal? When you open your traditional biology textbooks, five things are typically spoken of, two of which are actually not particularly important for classifying animals.
So, you may ask, what really is an animal? Here we present the most reduced form.
The animal: a multicellular entity that moves and eats other things. It also often begins life as a hollow ball "blastula" but there are exceptions. You, as an animal, as a human, can move to look for food. A mushroom cannot. The ability to move is a key distinction, and a control system is required to sense the external world quickly and coordinate such movement, thus almost all animals have a....
What does a nervous system then do? It is a computational and communicative system of specialized cells to quickly detect the environment, analyze it, and move the body in an appropriate manner and time. To do this, nervous systems have in general two principles of design: Neural Nets and Ganglia.
But as biology is famous for, there are exceptions to nervous system organization. A sea star has radial symmetry but does not have a neural net; it has a type of ganglia called a "nerve ring." And not all animals have nervous systems- specifically the two most basal animals: porifera (sponges) and placazoa (the tricoplax). All other animals though, be it jellyfish, clams, crustaceans, insects, fish, lizards, birds, mammals, and of course we humans, have nervous systems.
We can further organize nervous systems between bilateral animals that are vertebrate versus invertebrate. Vertebrates have a spinal cord encased in a bony spinal column and consist of the birds, amphibians, lizards, fish, and mammals (yep, those five are it). Invertebrates, a much larger and more diverse group, lack spinal columns, can be soft bodied or have exoskeletons, and consist of effectively every other type of animal (worms, insects, mollusks, crustaceans. etc).
A notable difference between vertebrate nervous systems and invertebrate nervous systems is the "degree of centralization." A cockroach, for example, has ganglia spread along along its nerve cord and a slightly larger ganglion in its head (its brain). This ganglia (brain) is much more enlarged in vertebrates, and the ganglia of the nerve cord (spinal cord) are smaller relative to the body.
What creature is the question mark? It is a "sea-squirt," a type of animal that is motile in its larval phase, but once it finds a rock substrate, it undergoes metamorphosis and sometimes dissolves its own nervous system in its sedentary form! The world of invertebrate neuroanatomy is full of strange secrets on the origin and function of nervous systems, such as.....Where did the Neuron come from?
Due to morphology and genetic studies, the neuron is thought to have only evolved once from a universal common ancestor, unlike eyes (6-7 times) or wings (4 times - pterodactyls, birds, insects, and bats). What creature then had the first nervous system? When did the 'first neuron' with electrical signaling appear? The simplest nervous systems are thought to be the nerve nets of the "Cnidaria" group, which includes anemones, hydra, and jellyfish. Clues may lie in these creatures.
The neuron only evolving once is now up for debate due to the recent genetic sequencing of comb jellies (Ctenophora). Comb jellies are sometimes popularly considered jellyfish but are a completely different type of marine creature. Notably, they are the largest life forms that swim with cilia along their bodies (like super size protists). The genetic studies have revealed that comb jellies have ion channels in their neurons that are distinct from all other animals' ion channels. Perhaps it developed neurons independently. New information becomes available every year.
Thus we leave you for now with the revelation that comparative neuroanatomy and physiology is an area rich for discoveries. How the nervous systems of jellyfish and anemones operate is virtually untouched. How neural nets function is unknown. And the secrets into the first nervous systems may be guarded by the comb jellies.....we await the unveiling.
 You can read about the famous experiments of Miller-Urey if you are curious into the question of abiogenesis and how living systems systems can emerge from primordial terrestrial chemistry.
 Planet-Hunter Astronomers are trying to identify ""habitable zone" planets that contain spectral evidence of water, considered a universal solvent of life. The sub-surface sea on Jupiter's moon Europa also is an intriguing candidate, as well as the strange "hydrocarbon" oceans on Saturn's moon Titan.
 Prions and isolated DNA fragments not withstanding.
 Biology Nerds - we know that the different types of multicellular algae (brown, green, red, etc) are classed into their own individual families - but to simplify things we leave them out for now and speak of only Plants, Fungus, and Animals.