From Tissues to Organs
With the advent of multicellular forms, specialisation of function seems inevitable. Tissues can be regarded as groups of cells with a common function. Organs can be regarded as groups of tissues with a common (overarching) function.
Even the simplest multicellular animals and plants on Earth have at least two cell types. Vertebrates have over 200 distinct cell types, which can be grouped into several organ systems.
The Volvox mentioned in the previous part is a good example. A hollow sphere of cells joined together by an extruded matrix of protein and carbohydrate, the Volvox colony has some differentiation into reproductive and non-reproductive cells.
From the Volvox the next logical step is to have an 'outside' lining and an 'inside' one. An interior chamber could be formed in order to digest foodstuffs. This strategy is employed by hydrae and other coelenterates like the sea anemone. (Coelenteron literally means 'gut space'). The cells lining the interior chamber are specialised to secrete digestive enzymes. The outer cell layers serve a protective and propulsive function (being capable of co-ordinated contraction). [Cells based on a matrix in contact with some external medium can be called epithelia or surface cells].
Within the space between the inner and outer cell layers lies the hydra's rudimentary nervous system. This internal environment is optimised for transmission of nerve impulses that control movement through the fresh water in which the hydra lives (see discussion below).
If we consider the embryological development of animals more complicated than the coelenterates, it becomes evident that all the tissue types that make up an organism arise from three 'germ layers': ectoderm, mesoderm and endoderm. In broad terms, sensory, "skin" and neural tissue arise from the ectoderm, structural and connective tissue from the mesoderm, and the bulk of the internal organs from the endoderm. Other aspects of development will be discussed in a later section.
Beyond a certain size threshold, simple diffusion of nutrients in and waste products out cannot efficiently meet the metabolic requirements of an organism. This size cutoff depends on the organism's metabolic rate, and the parameters that mathematically describe diffusion: diffusion rate = JA(dC/dx), where J is the permeability of a membrane, epithelium, tissue, etc. to the substance in question; A the cross-sectional area available for exchange; dC the concentration gradient; and dx the distance over which diffusion must take place.
Transport pathways arise to meet this need. At first there is fluid movement generated by pulsatile contractions of the various cell layers. Eventually a specialised system of pipes, cells and pumps appears to serve the function of nutrient and waste carriage.
Circulatory systems range from a simple cavity (haemocoel) whose contents are propelled around the organism to a complex circulatory system with hearts, capacitance, resistance and exchange vessels (veins, arteries and capillaries respectively). Larger plants rely on a system of fine capillary tubes in which fluid flow is driven by surface tension.
Gases required for respiratory processes require a parallel system of conduits. In small organisms a tracheal network enables gases to pass in close proximity to every cell, thereby reducing the distance required for diffusion. Larger organisms tend to have specialised gas exchange organs (gills, lungs) through which all of the gas carrying capacity available in the circulatory system is passed.
Nutrient uptake can be divided into two phases; digestion, where biological polymers are broken down into their constituent subunits; and absorption. It becomes evident that as creatures get larger, they must develop more efficient means of performing the functions outlined above, as the decline in energy expenditure predicted by Kleiber's law would suggest.
Kleiber's law and some interesting rules of thumb regarding scaling effects
In the 1930s an American veterinarian, Max Kleiber, noted that metabolic rate was proportional to an animal's mass raised to the 3/4 power. This relationship has been investigated in plants, animals and cellular organelles and would appear to be generally valid across 27 orders of magnitude (!). Other parameters that fit power functions include (but are not limited to) longevity, heart rate, number of offspring, generation time, and lung surface area. In general: variable = constant x masspower
The constants vary with the group of organisms concerned (birds, mammals, etc.)
Why are such relationships important?
For the purpose of this discussion, it allows us to guesstimate sensible answers to questions like "How much methane does this creature need per hour to survive?" or "What is the IR signature of a K'kree in ceremonial armour at laser rifle range?"
In more general terms the presence of power laws suggests that organisms are optimally adapted to get nutrients in and wastes out, and as they get larger the adaptations required change (and are generally more efficient).
Some (more) biological gearheadedness
- Metabolic rate is proportional to oxygen consumption.
Each litre of oxygen consumed (at standard laboratory conditions, 25 degrees Celsius and one atmosphere (101kPa, 14.5 psi, 760 Torr) pressure) liberates 4.82kcal or 20.15kJ.
In physiology, the respiratory quotient (R) is the ratio of carbon dioxide produced to oxygen consumed. Different energy substrates have different R values when oxidised:
- carbohydrates 1.0, 4.1kcal/g
- fats 0.7, 9.3kcal/g
- protein 0.82, 4.1kcal/g
- average diet R=0.8
A simple, if somewhat inaccurate rule of thumb could be as follows: One litre of oxygen at standard lab conditions is about 0.04 mole. This amount of all the reactants is required to liberate 4.82kcal/20.15kJ.
- Metabolic rate varies with the fourth power of absolute
temperature (living things behave like blackbody radiators).
Over the liquid temperature range of the solvents mentioned in the first post (113-433 K) a 216-fold range of values is therefore possible.
Creature core temperature (that of the most metabolically active tissues) must be more than ambient to permit radiation of waste heat. (Aside: The core temperature of birds and mammals ranges from 22-39 degrees Celsius with most clustered tightly around 37-38 degrees; hibernating mammals can get down to 3-4 degrees).
Creature mass is approximately volume X solvent density :-
|Solvent||Formula||Density of liquid (g/cc)|
Let us consider an example.
The PCs capture a 0.5kg 'methane-breathing' animal (about the size of a rat) on an icy world (average surface temperature 240 K). We shall assume the beast has a core temperature of 260 K.
Metabolic rate ~ (temp ratio)4
For convenience we shall use a rat's core temperature for this calculation:
metabolic rate ~ (260/310)4, or 0.49 that of a comparable mammal
metabolic rate in kJ/hour = 17.42 X (mass in kg)0.75 for mammals
0.49 X 17.42 X (0.5)0.75 = 0.59kJ/hr
This requires (0.59/20.15) L/hr of methane at 298K.
Using the ideal gas law, the volume of gas required at 240K is (0.59 X 240/(20.15 X 298)) = 0.024L per hour (24mLs).
N.B. The metabolic rate derived using the formula above is a basal one (sitting quietly, reading, etc.) Metabolic rate can be increased by a factor of eight with vigorous exercise (e.g. cross country skiing).
Just for completeness: what is the IR signature of our K'kree guard? K'kree mass upwards of 500kg. Let's say he weighs in at 600kg and behaves in physiological terms much like a terrestrial mammal.
Metabolic rate = 17.42 X (600)0.75 = 2111.8 kJ/hour = 586.6J/sec or 586.6W. In DSR terms, the K'kree has a passive IR sig of -2.5.
A short list of functions required of the tissues/organs of most living things might look like this:
- Nutrient absorption
- Transport of nutrients and waste products
- Waste excretion
- Maintenance of a stable internal environment - homeostasis.
- Monitoring of external conditions that may impinge on (iv) and means of adapting to any changes that may take place.
- Structural support
i. Nutrient absorption
In terrestrial animals, this function is served by a digestive system which processes foodstuffs, breaking the constituents into smaller molecules to facilitate absorption. These absorbed compounds are either stored or distributed to working tissues as required.
In man, food is chewed to increase its surface area; saliva is secreted to moisten the bolus and begin digestion of carbohydrate; the food bolus is then propelled into the stomach, where acid and pepsin are released to facilitate protein digestion. Food is then propelled into the small intestine where it is exposed to digestive enzymes released from the pancreas and bile, which helps emulsify fats.
Absorption takes place predominantly in the small intestine; waste material is dehydrated in the large intestine and stored pending its expulsion.
Other animals have variations on this basic theme e.g. multiple stomachs to facilitate the digestion of cellulose in cattle and other ruminants.
Absorbed material is conveyed to the liver - the body's metabolite processing centre and either stored (carbohydrates -> glycogen), shipped (fats to fatty tissue, sugars to the brain, etc.) or spent. The analogous structures in plants are the root system and leaves.
ii. Transport of nutrients and waste products
There are two parallel liquid transport systems that are used to move nutrients and wastes to and from the tissues in most Earthly animals. These are the circulatory systems that move blood (gas transporters, many proteins and low molecular weight solutes) and lymph (larger proteins or less soluble solutes e.g. some fats).
Creatures that rely on liquids or solid reactants (e.g. sulphur or hydrogen peroxide 'breathers') may have a combined digestive/respiratory tract, as the volume of reactant required is small compared to gases.
Very small (10 gram mass range or less) gas-breathing creatures will have a network of interconnected air pores (tracheal network) to enable gas exchange by diffusion. The circulatory system in insects is largely separate from the tracheal system, serving mainly to shuttle nutrients from the gut. Larger gas breathers will require a specialised lung or gill arrangement with sufficient area to permit gas exchange (e.g. 300 million alveoli in man have a total surface area of 70m2).
A pressure gradient needs to be generated to draw air into the lungs, which requires distensible lungs and a sealed muscle lined compartment to hold them in. [The arrangement described for the K'kree in G:T Aliens Module 2, where a muscle lined windpipe and the 'gut booster' mechanism (diaphragmatic pump) helps inflate two sets of lungs not confined in a ribcage is plausible but inelegant].
Specialised transport compounds will arise to convey reactants in the case of the gaseous ones in most cases ; there is not enough gas dissolved in a solvent to meet metabolic needs until very low temperatures are attained (e.g. some Antarctic fish live in waters that range in temperature from 2 to -2 Celsius; their metabolism is so slow that they only need dissolved oxygen). Haemoglobin is the oxygen transport compound in a wide range of terrestrial life; it relies on an iron atom to co-ordinate with molecular oxygen (and is the reason why blood is red). Other creatures use copper based compounds (e.g. haemocyanins in some crabs).
[Aside : For fans of funny coloured blood: sulphated haemoglobin is brown, reduced (met)haemoglobin is blue. Copper-based compounds will be blue, green or red; Cobalt-based compounds will be red, pink or blue.]
Blood, whatever its chemical constitution (nitric acid as solvent?), requires at least one pump to maintain its circulation. Multiple hearts are frequently encountered in science fiction; as long as the pumps are co-ordinated so as to guarantee forward flow, all is well (as anyone who has worked with circulatory assist devices in people will attest). Terrestrial life tends to have one primary heart and an 'informal' system of pumping driven by surrounding muscles - vasomotion. Vasomotion is crucially important for lymph flow.
iii. Waste excretion
Waste flows are of two types:
- substances that can't be processed by the digestive system
- byproducts of metabolism
In man, the primary byproducts of metabolism which aren't dealt with by the digestive system are carbon dioxide, urea and the so-called 'fixed acids'. Carbon dioxide excretion takes place in the lungs and accounts for about 85% of the acid load removed from the body each day. Urea and the 'fixed acids' (e.g. sulphuric, phosphoric) are products of protein metabolism and are eliminated through the kidneys.
iv. Homeostatic functions
The term homeostasis was coined by the French physiologist Claude Bernard. It refers to the maintenance of constant conditions within an organism. Wide changes in solute concentrations, acid-base balance and temperature can have sweeping effects on metabolism. Some system of feedback control is required.
Feedback loops are a common motif in physiology:
- interpret sensory inputs
- determine appropriate response
- relay instructions to effector systems
- new stimulus
There are two broad classes of control system: rapid and slower-acting. The nervous system relies on electrical (currents generated by the redistribution of ions across cell membranes) and chemical transmission (release of specific compounds - neurotransmitters - at synapses which may be between nerve cells as well as between nerve cells and those of effector organs) of information in order to rapidly adapt to a changing environment, forming the substrate for one of the faster control systems.
Let's consider an example. Blood loss leads to a decrease in circulating blood volume. This is sensed by pressure sensors in the carotid artery and right atrium of the heart. Data is relayed to the vasomotor centres in the brain stem which leads to stimulation of the heart (to beat faster) and to the smooth muscle of the capacitance and resistance blood vessels in order to preserve blood pressure and cardiac output. Hormones are produced by the kidney and other glands to initiate regeneration of the lost blood cells, redistribution of fluid to maintain blood volume, etc.
Hormones are chemicals that mediate longer term changes in organ function. These are under negative feedback control (so that when the target condition is achieved the secretion of the stimulatory or inhibitory hormone is decreased). The endocrine glands are the primary example of a slower-acting control system. The kidney exerts a fine control over solute concentrations via retaining or losing water and acts as a secondary controller of acid-base balance.
Depending on the diameter and resistance of a nerve fibre, messages can be relayed at speeds up to 70 metres per second. Are there any other methods of information transmission that could form the basis of a rapidly adaptive control system?
Another way of utilising electrical currents or light may be to have a network of chlorophyll or haemoglobin-like substances forming 'wires'. Electrons are shuttled from metal ion to metal ion (chlorophyll has a central magnesium ion; haem, iron) with great rapidity. Such a system could develop in environments where metallic elements are more abundant than on Earth. 'Biological' optical fibre or superconducting links would be plausible alternatives for silicon based life. The propagation of pressure waves through a network of fluid filled tubes is yet another alternative. Problems would arise at intersections and the like, as the waves would interfere with each other. 'Fluid logic' switches are a possible solution for the latter problem.
An area of current controversy concerns plants. Hormone mediated control is well described in plants. Recent molecular biology work has isolated genes for several transmembrane proteins which could generate potential differences via the movement of ions, similar to animal nervous and muscle tissue. Can the circulatory system of a plant double as a conduit for information transmission?
Another very important homeostatic function is that of immune surveillance. The organism needs to protect itself from cancerous, microbial and parasitic threats. Recognition of 'self' and 'not-self' cells is an ancient feature; most invertebrates have a well-developed immune system.
Surveillance is conducted, as always, at borders - in the gut, respiratory tract and the skin - and along major highways (the circulatory system). In terrestrial life, the immune system is closely associated with tissue repair functions. This may or may not be a 'universal' (Referring to the idea that all biospheres will have common 'milestones of development' e.g. photosynthesis, aerobic respiration, multicellularity, nervous systems, flight and perhaps intelligence with sufficient time).
v. External sensors
The outside world changes. Organisms need to be able to meet the challenges of the environment in which they live. Broad classes of sensors include:
- Electromagnetic: the eye is the most obvious example.
Proto-eyes probably began as areas of skin containing light sensitive
cells; an excellent account of the evolution of eyes is found in Richard
Dawkin's 'Climbing Mount Improbable' (chapter 5). It has been estimated
that eyes evolved independently at least forty times during life's
There are two broad types: compound and camera-like organs. The former are made up of numerous light guides (ommatidia); each light guide serves a small group of photocells. Compound eyes are limited in their useful size by the wave properties of light, so they are now found pretty much exclusively in small insects. The wavelengths of light sensed by eyes varies from 400 (near ultraviolet) to about 800 (near infra-red) nanometres. Having eyes sensitive to shorter wavelengths is very unlikely due to the extreme penetrance of these radiations, and their ability to disrupt chemical bonds within biological molecules; having eyes sensitive to vastly longer wavelengths is impractical due to the large size requirement for the sensor (and in some cases the lack of local sources for the radiation e.g. millimetre and radio waves).
Eyes are extraordinary sensors, capable of detecting light across twenty orders of magnitude of intensity (consider the difference between a moonless night vs. staring into the sun).
Most animals can sense infrared radiation (via heat receptors in the skin). Certain reptiles e.g. the rattlesnake have 'infrared eyes' or pit organs which can discriminate between very fine temperature differences, on the order of 0.1 degrees Celsius.
Many marine animals can sense changes in the faint potential differences between their bodies and the salt water they live in. Sharks are a notable example; they possess organs just above their mouths - the ampullae of Lorenzini - which enable them to track prey using this modality alone!
- Pressure waves: the ear is the most notable example. The
human threshold of audibility corresponds to a pressure of 2X10-5
Pa (atmospheric pressure is 1.01 X 105 Pa) or a power level of
10-12W/m2. The ear is sensitive to power levels across a range
of seven orders of magnitude(!).
Bats and dolphins use sonar to track prey and communicate with each other at ultrasonic (above the human limit of audibility ~20kHz) frequencies up to 150kHz. Most organisms have pressure receptors in their skin (the sense of touch).
- Chemical receptors: taste and smell depend on recognition of various (extrinsic) chemicals. Substances are sensed as bitter, sweet, etc. on the basis of their molecular shape and charge distribution. Wide differences in acuity between animals are well recognised (e.g. dog vs. man). Other forms of important chemical receptor are pain receptors (which detect local release of inflammatory mediators in response to injury) and chemoceptors in the blood vessels and brain which detect changes in carbon dioxide and oxygen partial pressure and hydrogen ion concentration.
- Accelerometry: acceleration detection is essential for co-ordinating movement and orientation to the local gravitational field. In man, the sensors are located in the semicircular canals in the middle ear (vestibular apparatus). Three fluid filled canals at right angles to each other contain small calcium carbonate chips (otoliths) which amplify any movement in the fluid, which is sensed by 'hair cells'. The cilia of the hair cells are moved by the shifting fluid and the data is processed by the brain stem. The analogous structure in the fish is called the otolith.
Animals need to move around to 'eat and not be eaten'. The essential requirements are:
- some means of converting chemical to mechanical energy
- a system for co-ordinating and controlling the 'movement mechanism'
Muscle tissue converts chemical energy to mechanical energy. The underlying mechanism of contraction and relaxation is a ratcheting or sliding of protein filaments over one another, driven by the ATP produced by the breakdown of glucose.
The two proteins that make up the bulk of the filaments, actin and myosin, are found in the cells of most living things. Actin is an important part of the 'scaffolding' that maintains cellular shape and the position of organelles - the so-called cytoskeleton. This forms the basis of cellular locomotion and contraction in non-muscle cells too.
There are two broad categories of muscle tissue: striated (where the contractile filaments are arranged in parallel chains) and smooth (where they are not). Striated muscle is capable of forceful but relatively short duration contractions. Smooth muscle is found in the walls of blood vessels and hollow organs. It is capable of sustained contraction.
The harnessing of chemical or thermal gradients to cause expansion or contraction of a tissue might be possible alternative mechanisms. An example could be a substance that contracts with an increase in acidity or temperature and relaxes when conditions return to baseline.
Most locomotion systems rely on lever (e.g. legs, wings) or spring effects (e.g. in the legs of a hopping or jumping animal - also important in running). Other muscle-based systems include hydraulics (e.g. the legs of the jumping spider rely on rapid fluid shunting for the jumping action) or jet propulsion (e.g. forceful contraction of a swim bladder in the flying squid and octopi).
Co-ordination and control: It has been said that proprioception is the 'sixth sense', monitoring the position of limbs, angulation at joints, and body orientation. An animal's nervous system processes large amounts of sensory input from accelerometers and proprioceptors from moment to moment to prevent a trip or stumble even at rest.
Recent work has demonstrated that an appropriate network of feedback loops - a 'nervous net', if you will - can be constructed without large amounts of computing power (which was suspected from early animal experiments where quadriplegic animals could be made to walk with the application of small currents to the spinal cord). This has somewhat simplified the lives of walking robot designers. In vertebrates, the spinal cord can generate simple, almost stereotyped movement patterns. Various areas of the brain (e.g. the motor cortex and cerebellum) exert fine feedback control over the spinal neurons.
How many limbs? Insects have six legs, arachnids 8, and other arthropods and worms several hundred. In these organisms, there are other structures (antennae, mouthparts, palps, pincers, etc.) which are modified legs (to be discussed below). Vertebrates have four limbs (fins, wings, legs) which may only be present in vestigial form (e.g. hindlimbs in whales). There is obviously a wide variation in possible leg number. What factors influence how many eventually appear?
- creature size : with increasing creature mass, legs have to become bigger to support the body (an aspect of the 'square-cube' or 2/3 power law)
- speed vs. stability : a creature may have to move quickly to catch prey (or avoid becoming prey). Research into the dynamics of leg-based propulsion has shown that you can move quickly (but won't be particularly stable at rest - dynamic stability) or be stable at rest (and not be able to move quickly - static stability).
- energy requirements : varies with leg number and their mass relative to the mass of the creature's body. Millipedes and centipedes use lots of relatively small legs to get around.
Larger creatures tend to have fewer legs ; while there is more redundancy with having more legs there is a bigger trade-off in maintaining them. Storing energy in the elastic tendons and muscles of legs or wings keeps overall power consumption down. This is important in running and hopping gaits. Tails - extensions of the vertebral column protecting the spinal cord - may be useful in manipulation, balance or propulsion, depending on the creature's habitat.
vi. Structural support
Scaffolding is required to act as anchorage points for muscles and tendons and to support and protect vital organs. This requirement is somewhat reduced in the case of small or aquatic creatures (both exist in a relatively weightless environment). Materials used for this purpose vary widely throughout living things. They can be grouped as follows:
- carbohydrate polymers: cellulose (plants), chitin (insects). Strong, relatively lightweight, water insoluble. Wood is a cellulose composite with fibres running in laminae in which the fibres of successive layers are at right angles to each other for additional structural strength.
- carbohydrate-protein composite: cartilage (e.g. sharks). Cartilage is made up of long carbohydrate chains with a variable collagen and water content.
- protein-salt composite: bones, teeth - collagen chains form the flexible component ; these fibres are contained in a matrix of calcium salts, primarily phosphates and hydroxyapatites.
- deposited salts: e.g. glass skeletons of diatoms; corals (limestone); shells of molluscs, snails, etc. are calcium salts and silicon oxides.
In general terms, materials used will be either biomolecules or compounds readily available from the local environment, or combinations. Reducing salts to their constituent elements is energetically expensive and high concentrations of metal ions are usually toxic to most Earthly life (so a creature that builds steel shells or deposits gold dung is very unlikely). Increasing the concentration of minerals in an area is certainly possible (a coral reef and limestone, for example).
"Organisms are DNA's way of making more DNA." What strategies have living things employed to perpetuate their lineages?
- Binary fission: single celled organisms typically reproduce by mitosis. The genetic material is duplicated and the cell divides in half. Relatively uncomplicated stuff; the generation time for some bacteria is on the order of minutes.
- Sexual reproduction: In most animals and plants, specialised reproductive cells (gametes) are produced which contain half of the parent's genetic information. This process is called meiosis. In some organisms, the gamete forms are the form of the organism normally seen - these combine transiently to produce gametes with a new genetic makeup. Bacteria and other single celled organisms occasionally reproduce sexually.
By way of background : genetic material is typically bunched into chromosomes. For example, human beings have 46 chromosomes in 23 pairs. The gametes have 23 chromosomes (one is chosen at random from each pair). So in theory the possible number of different sperms or eggs one could produce is 223 or 8388608. The idea that 'each of us is a long shot' is a pretty valid comment.
Why bother with this shuffling process? What are the benefits of sex? Several theories have arisen. The common element to all of them appears to be that recombination of genetic material appears to serve two functions: to limit the damaging effects of harmful mutations, and to generate new genes which may be of use in later generations.
How is sex determined? What alternative models for sexual reproduction are there? Sex is determined in several different ways throughout the animal kingdom. In mammals and birds, the sex chromosomes are paired. Whether one is male or female depends on whether one has two copies of the same chromosome or not. (So, in man XX - female, XY male. In birds, there are several different chromosomes W, Z, X; different species vary in whether WZ is male or female, say). Other species have different arrangements - reptile sex is sometimes dependent on temperature or hormonal triggers determining what genetic material gets expressed.
In a draft of the rules for one version of Traveller, the following useful classification is used:
- Solitaire: this could apply to binary fission or parthenogenesis. The latter occurs commonly in reptiles e.g. snakes. Two gametes are joined together, or a clone of the parent is produced.
- Dual: the situation with which we are all familiar.
- EAB: egg producer, egg activator, egg bearer. Normally producer and bearer functions are combined, sometimes egg activators [males] carry the eggs (e.g. Surinam toads). No earthly examples of complete separation spring to mind.
- FMN: female, male, neuter - e.g. social insects. Males are produced transiently to fertilise a female that produces all the offspring ; the vast majority of the community is incapable of reproduction.
- Multi: multiple sexes required for reproduction. No earthly examples.
Regulation of fertility: Typically most animals are regulated by hormonal clocks. Females have estrus cycles where they are fertile at periodic intervals. Males have similar cycles for sperm synthesis but often continually produce sperm.
In the situation of social insects, the dominant female may suppress sexual differentiation in prospective neuter forms at their birth, or throughout the lifecycle. A useful rule of thumb is that generation time is proportional to organism length raised to the 1/4 power. So a bacterium 10-6m long has a generation time measured in hours ; a 100m tall tree has a generation time of a century. The number of offspring is inversely proporational to size.
Reproductive mechanics - the simplest division is into egg-laying (oviparity) and live birth (viviparity).
Development and body plans
The history of life on Earth suggests that body form becomes tightly conserved once a lineage is established. For example, all vertebrates have a head at one end of the spinal cord and four limbs. How does a fertilised egg grow into a creature composed of billions, if not trillions, of cells? The biochemical bases of development have not yet been fully described. However, the available evidence suggests that cells differentiate in response to positional cues mediated by chemical gradients. As an embryonic organism develops, the cells remember their position under the influence of successive rounds of mediators. Different genes are expressed in response to the variations in concentration.
DNA binding proteins are responsible for varying patterns in gene expression, their synthesis regulated by the mediators that the cell is bathed in. It has been found that one form of DNA binding protein belongs to a family whose DNA sequence has been very tightly conserved through evolution. Mice, men and worms all have the same basic pattern generating mechanism that regulates which cells develop into the various body segments (e.g. head, thorax, abdomen, limbs).
These binding proteins are called homeotic selectors. A number of elegant experiments in the fruit fly have demonstrated bizarre effects from simple mutations in the homeotic selector genes : flies whose antennae grow as legs, double ended flies with two heads or abdomens, etc.
Looking back into the fossil record, one can see that a huge range of potential 'ancestors' were generated in the Cambrian 'explosion' where life attempted to exploit all niches available to it at the time. The Ediacaran flora and fauna and other fossils found in the Burgess shale look decidely strange. As Steven Jay Gould attests, if things happened a little differently, we wouldn't be here; intelligence may or may not have arisen in another lineage, and the variety of animals and plants present today are one set of a vast number of potential flora and fauna.
In summary, a few basic lineages of animal or plant will be found on a life-bearing world. Members of a lineage may be vastly different in size, etc. but they will have common features:
- Symmetry: front-back, top-bottom, left-right axes are the most likely ones to develop. Vertebrates and invertebrates have only one spinal cord or central set of ganglia. Other creatures display multiple axes of symmetry e.g. jellyfish and starfish. Instead of front-back, left-right axes they have multiple axes (radial symmetry). Asymmetry is unusual; an example might be bottom dwelling fish like flounder and plaice with their offset eyes. The number of axes of symmetry may reflect the distribution of the nervous and digestive systems. Multiple brains are unlikely, but multiple 'spinal cords' are not.
- Segmentation: different regions of the body contain tissues with different functions; for example head, thorax and abdomen in most animals. Worms and crustacea are made up in part of repeated segments with slight variations in form e.g. a lobster's abdomen.
- Endo or exo-skeleton: structural support and fulcra are needed to anchor muscles and tendons to. The materials used may vary slightly (e.g. cartilage in sharks vs. calcified bones in bony fish) but have many common components (collagen in the case of our fish). The upper bound for size for exoskeletons (in one g with carbon compounds) currently belongs to an extinct arthropod whose remains were recently excavated in Europe. Picture a centipede 2m long and about 0.5m in diameter! [The largest living arthropods, spider crabs, have bodies 0.5m in diameter and a leg span of up to 1.5m].
- Warm or Cold-Blooded?: does the creature attempt to maintain a constant core temperature or not? Smaller variations in core temperature have less adverse effect on acid-base balance and metabolic activity. The tradeoff is that it costs energy to maintain a constant temperature.
- Coverings: this is a function of the creature's habitat, which we will discuss in the next part.
Next Part: Ecology, Environment, and Evolution