I plan to write a series of posts on animal research. The idea came about when I was thinking about the way we have been told for years not to be anthropomorphic. There is an opposite word, zoomorphic : “The tendency of viewing human behaviour in terms of the behaviour of animals, analogous to anthropomorphism, which views animal behaviour in human terms.” This idea has also been frowned upon. Why are we cautioned to avoid understanding animals by seeing them using human concepts and understanding human by seeing them using animal concepts? These restrictions are slowly losing ground but they still appear in our books, magazines and newspapers fairly regularly. So this series is on comparative neurobiology.
Comparative Neurobiology 1: All animals are built on the same plan
A good place to start is near the beginning, before active animals. (It was active animals that required the development of a nervous system.) Neither yeasts nor sponges have nervous systems but they have much of the chemical machinery to make a neuron and its synapses. They do not use these chemicals together to make a synapse but use them separately for different tasks. When animals with nervous systems arose they already shared these basic chemical building blocks. All animals are inheritors of the chemical endowment for a nervous system to use and modify as needed by their way of life.
- Jon Lieff has some interesting blogs on this including (see below). “The synapse first appeared in evolution approximately 700 million years ago, with the first circuits of neurons soon after. But, the proto-synapse occurred over a billion years ago in microbes.” Synapses are very complicated (exceptionally so, I would say) but their components existed for other uses.
- “Mechanisms enabling one cell to influence the behavior of another almost certainly existed in the world of unicellular organisms long before multicellular organisms appeared on earth. Evidence comes from studies of some present-day unicellular eukaryotes such as yeasts.” (Alberts 1994).
- Of course, there have been modifications over all these years. “We studied around 600 proteins that are found in mammalian synapses and were surprised to find that only 50 percent of these are also found in invertebrate synapses, and about 25 percent are in single-cell animals, which obviously don’t have a brain….The number and complexity of proteins in the synapse first exploded when muticellular animals emerged, some billion years ago. A second wave occurred with the appearance of vertebrates, perhaps 500 million years ago.” (Emes 2008) This is not as clear cut a progression as it may appear – it may not even be a progression. It may be that if they had started with all the proteins in invertebrates’ synapses, they might have found 50% of them in mammals and a slightly different 25% in single-celled animals.
The similarity in proteins implies a genetic similarity. The take-home here is that there is a very strong family resemblance across all animal nervous systems even if they may look different on the surface.
How similar are all vertebrate nervous systems? All vertebrates have a spinal cord and at the front end of the spinal cord the brain develops. Three areas appear in early embryos: fore, mid and hind portions of the brain. The same ventricles appear. Different parts grow at different rates so that the end product looks different but they all start with a very similar basic architecture shared by fish, amphibians, reptiles, birds and mammals. This implies that the developmental signals prompting cells to migrate, differentiate and connect have a strong family resemblance, a conserved genetic plan. What the vertebrate scheme seems to be is that: the fore-brain (cortex, thalamus, basal ganglia and some other structures) receives sensory information and has the highest integration of information; the mid-brain coordinates reflex responses to sensory information; the hind-brain has reflex control of things like respiration and heart rate. But, for instance, the cerebellum is important to coordination of sensory, motor and cognitive dexterity in many vertebrates, so the hind brain is not excluded from complex activity.
For many years it was thought that the neo-cortex was found only in mammals but it is now known that birds and reptiles have similarly functionally connected neurons to those in the neo-cortex. In other words the non-mammals can make the similar connections to do similar cognition as mammals but with a structure that does not look like the neo-cortex. “Both the mammalian neocortex and a structure in the bird brain called the dorsal ventricular ridge (DVR) originate from an embryonic region called the telencephalon. But the two regions mature into very different shapes, with the neocortex made up of six distinct cortical layers while the DVR contains large clusters of neurons called nuclei.” (Dugas-Ford 2012). All vertebrates share the same fundamental developmental plan for the brain.
The brains of mammals are very similar. Mice for instance make good laboratory subjects because their brain are close enough to our own to mimic our diseases of the brain. Genes can be transferred from humans to replace the homologous genes in the mouse and the human genes work well in the mouse. Can there be a better test of physiological similarity? “New mouse strains engineered to express human genes related to mental disorders are being developed under a recently-launched grant program from NIMH’s Division of Neuroscience and Basic Behavioral Science. The new models are designed to help scientists understand the molecular workings of variations in genes that may predispose for – or even help protect against – illnesses like depression, bipolar disorder and schizophrenia. They will also explore how duplications or the differences in the amount of genetic material affect brain function, and how genes influence response to treatments. The new studies follow a spate of recent discoveries using such mouse models to replicate features of mental and developmental disorders.” (NIMH 2008)
Of course primates, particularly great apes and very particularly chimpanzees and bonobos are extremely similar to humans. Our brains are not identical but they share a great many fine details. There needs to be some caution in assuming that primates will always be closer to us than other animals. When we look at whales, dogs, elephants, bats, crows, parrots we find that there are some striking similarities with us but I will discuss these in another posting that looks at behavioural similarities.
We have only a few million years of separation from chimpanzees and bonobos. These three species are so close that if they were almost any other group, they would be put together in one genus. For example the Canus genus contains 10 species: various wolves, dogs, jackals and coyote – that is certainly a wider range than the 3 species: human, chimpanzee and bonobo which are divided into Pan and Homo.
So given the anatomical, biochemical, physiological, developmental and genetic similarities between us and other animals, why should we not look at them to understand ourselves and look at ourselves to understand them? The great many experiments and observations that have been done on the brains of other animals have been invaluable and this research will continue – because anthropomorphism and zoomorphism are useful legitimate tools.
Image: Jan Breughel the Elder – The Entry of the Animals into Noah’s Ark – Google Art Project
Alberts B, Bray D, Lewis J, et al. Molecular Biology of the Cell. 3rd edition. New York: Garland Science; 1994. General Principles of Cell Signaling. Available from: http://www.ncbi.nlm.nih.gov/books/NBK28317/
R Emes etal. Evolutionary expansion and anatomical specialization of synapse proteome complexity. Nature Neuroscience 11, 799 - 806 (2008) http://www.nature.com/neuro/journal/v11/n7/full/nn.2135.html
J Dugas-Ford etal. Cell-type homologies and the origins of the neocortex. PNAS October 16 2012 vol.109 no 42 http://www.pnas.org/content/109/42/16974
NIMH press release, Mice Expressing Human Genes Bred to Help Unravel Mental Disorders http://www.nimh.nih.gov/news/science-news/2008/mice-expressing-human-genes-bred-to-help-unravel-mental-disorders.shtml