POST 6 – 18-02-2014
In the last POST on the cosmic saga I said that the next would be on IRRITABILITY. It is a rather technical usage and just means the ability to be stimulated.
Using this term has caused a bit of a stir so I will revert to “sensitivity”.
- SENSITIVITY TO THE ENVIRONMENT
- PRIMITIVE CELL
- EARLY CHANGES
- LATER PLANTS AND ANIMALS
- MAMMALS AND BIRDS
- 1. SENSITIVITY TO THE ENVIRONMENT
Each primitive cell was a separate individual living item with a cell wall and the ability to conduct chemical reactions (metabolise – obtain and use energy), to grow (develop and repair), to produce progeny (reproduce), and to respond to stimuli.
Even the most primitive organisms were intimately involved with their environment and had to survive there. This meant they had to have some primitive awareness of change in that environment. Some sort of signal must have been received by the organism and some response mounted by it. A primitive stimulus-response mechanism providing feedback was needed. How this primitive mechanism evolved into our nervous system is a fascinating study.
- 2. PRIMITIVE CELL
Charles Darwin in his book On the Origin of Species, proposed a single original cell saying, “Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed”.
The first cell, sometimes called the Last Common Universal Ancestor (LUCA) had to be able to survive in an environment that was conducive to its survival or it would not have existed at all. A change in that environment may have been beneficial, harmful or neutral. The possible responses of the cell to harmful stimuli were limited. It could have slowed down its metabolic processes, toughened its cell wall (perhaps simply as a result of drying out), or gone into a temporary quiescent or latent phase. Or, of course, it may have died and that was probably the most common response to changes in the environment. The pressure was on to develop more alternatives and later evolution favoured things like mobility ( so it could move away from noxious stimuli), more elaborate protective mechanisms like tough shells (as in spores of bacteria and fungi, oocysts in coccidia, or seeds and nuts in plants), or claws, horns and other defensive or offensive weapons.
The primitive response enabled the organism to react to the environmental stimulus. The evolution of this response led to the more sophisticated stimulus-response mechanisms we see in higher animals and to the arms race involved when the predator evolves greater speed, agility or cunning and the prey evolves greater speed, agility or cunning to defend itself and so on … and on ….
- 3. EARLY CHANGES
Some early cells benefitted from combining with other cells to form multicellular organisms where mutual benefit to component cells became a benefit to the whole organism. In the multicellular organisms there soon developed a differential function of some cells which led to specialisation, with some cells dedicated to separate functions such as digestion or reproduction and some to signalling – sending messages to the nervous system and back to the muscles
The stimulus response mechanism seen in an early form in primitive cells is the basis of our nervous system. A plant or animal cell could benefit from having developed an improved ability to be stimulated by (or be sensitive to) its environment as well as by having developed an ability to respond to those stimuli.
Even a simple organism, presented with a change in environmental circumstances has some, though limited, alternatives. It did not have any mechanism to make decisions, but there is the appearance of some choosing mechanism that enabled those progeny that could survive to do so. Surviving progeny clearly already had the appropriate pathways to allow survival. A cell that survived had mechanisms that allowed it to respond appropriately to changes in the environment and could muster up an appropriate response to stimuli. This may not look like a voluntary action but there is something like a primitive expression of a preference or a choice going on.
Quorum sensing in bacteria is a good example of an early mechanism that is a stepping stone towards more sophisticated stimulus-response mechanisms. Some bacteria excrete chemicals into their surroundings that act as signals to their neighbours. The receivers of these signals respond by taking clear actions which do not occur in the absence of the signals.
Quorum sensing has been seen in number of bacterial species but was first observed in Vibrio fischeri, a bioluminescent (light-producing) bacterium that lives in the light-producing organ (photophore) of the Hawiian bobtail squid. When the bacterial cells are free-living, the trigger chemical (auto-inducer) is at low concentration, and cells do not luminesce. However, when they are highly concentrated in the photophore (about 1011 cells/ml), the enzyme luciferase is induced, leading to bioluminescence.
Bacteria can talk to one another in this way and co-ordinate their behaviour.
“As environmental conditions often change rapidly, bacteria need to respond quickly in order to survive. These responses include adaptation to availability of nutrients, defence against other microorganisms which may compete for the same nutrients and the avoidance of toxic compounds potentially dangerous for the bacteria. It is very important for pathogenic bacteria during infection of a host (e.g. humans, other animals or plants) to co-ordinate their virulence in order to escape the immune response of the host in order to be able to establish a successful infection”.
- 4. LATER PLANTS AND ANIMALS
Evolutionary led to the eventual arrival of more sophisticated, and more complex, organisms that were able to detect or sense, a variety of environmental changes (stimuli) and also to respond appropriately.
They evolved other ways to send and receive signals and how to distinguish between different demands upon the available responses. As cells evolved such responses they were adding mechanisms that could be stepping stones into later adaptations. With added complexity, adaptations such as multicellularity, specialisation of structures (cells, tissues, organs and systems) and functions (mobility, thermoregulation, homoeostasis) evolved.
The ways in which living organisms adapt to their environment are a separate subject, but for this exercise it is clear that natural selection is the way to survival through the evolution of an elaborate stimulus-response mechanism.
Plants evolved mechanisms allowing them to react to environmental stimuli including gravity, light, temperature, moisture, injury and disease. Some plants are sensitive to touch and will close up their leaves when stimulated by something such as an insect walking onto its surface. Plants also appear to have some sort of proprioception or sense of the relationship of their own parts to one another.
In addition, animals evolved mobility and the specialised senses and the big difference being the brain.
- 5. MAMMALS AND BIRDS
Non-human animals and birds have a nervous system appropriate to their needs. Some of them have evolved quite sophisticated memories and show learning abilities that are indicative of evolutionary responses that are suitable for their survival in the environment. Evolution has gone so much further in the human nervous system where the feedback operations are partly unconscious and partly under our voluntary (autonomic) control.
Some animals seem to have preferences and have the ability to make choices. The classical conflict is the choice between flight and fight. Does the baboon make a choice when it hears a sound in the bushes (which could be a predator or a harmless fellow baboon), or does it have an automatic weighing-up process that gives it the best possible outcome? A predator would require an immediate flight response without further checking while a fellow baboon would need a second look to see if it is a stranger or not.
One of the best examples of an isolated but advanced animal ability is the bird practicing eight separate actions to get at a food reward as seen in the video of this smart crow.
When the central part of the nervous system recorded the inputs from the sensory organs the organism was able to store the information and to retrieve it when needed was a clear advantage in the competitive life it lived. The sensory system evolved a mechanism which allowed the animal to sense and to be aware of the information that was stored in the memory and awareness, including self-awareness, emerged. Some animals show primitive self-awareness. Most animals are unable to recognise themselves in a mirror but elephants are able to do so.
In humanity that level of self-awareness became what we call consciousness.
As the evolving organism became more complex, the ability to perceive preferences and to make a choice came to have more significance even though it was, and is still, an ability limited by the constraints imposed by biology, by habit and by the limited range of realistic options. In fantasy or in imagination we can overcome these limitations and that opens other doors to the mind.
A very early single celled organism can find some things in the environment attractive, some neutral and others repellent. This sensitivity or irritability was beneficial to survival and soon was discernible when the organism exhibited preferences which implied giving value to some things in the environment over others. The first sign of an ability exhibit a preference and to make a choice emerged.
This ability to have preferences and to choose, flowered as other abilities evolved and ultimately survives as ethics, the choices we humans make about our behaviour. The things we have acquired through our biological heritage modified by what we acquire through our cultural associations and learn during life’s experiences all inform our beliefs and behaviour. Some say we have no free will but it seems clear that, as even primitive animals had mechanisms for making choices, so we also have a limited ability to do so within the constraints. Perhaps free will is not an absolute and we are deeply influenced by our biology and our history but it seems to me that there is some limited room for responsible choice.
This means that we are able to make commitments and keep to them – albeit with varying degrees of attention, determination and success.
Thus we have come a long way from LUCA to love.