Life As We Know It!

In order to look for intelligent life on other worlds we need to be clear about what we actually mean by "life", and look carefully at our own history to categorise the factors that contribute to the evolution of an intelligent technical species. We will then be able to speculate more scientifically on the relative abundance of extraterrestrial intelligence (ETI).  Despite the incredible biodiversity evident on Earth, both now and in the past, biologists can identify patterns and behaviours shared by all of terrestrial biota. These are:

• highly ordered structure (negatively entropic)
• based on highly reactive chemical processes
• chemically unstable (i.e. will decay into more stable compounds after death)
• capable of growth
• capable of replication
• capable of repair
• continual demand for energy
• evolving

Non-living systems can show some, but not all of the characteristics listed above. For example a fire can grow, replicate itself (by sparks), is highly reactive and demands fuel, but has no ordered structure. Computer viruses are ordered and duplicate themselves, but are not reactive or chemically unstable nor do they have a continual demand for energy.
Not only is life itself chemically unstable, but it creates an environment around it which is also full of unstable compounds. For example oxygen is a very reactive gas, and without its continual production by photosynthetic organisms there would be no free oxygen in Earth’s atmosphere.
We may use the working definition of life as any system that obeys all of the above properties. We see life as a dynamic and evolving process whereby matter is taken into a system and used to assist the system’s growth and reproduction with waste products being expelled.
This idea of persistent self-organisation is at the heart of James Lovelock’s "gaia" hypothesis.

The Chemical Basis of Life

All life from viruses to whales requires carbon and liquid water - the chemistry of these two substances providing the versatility needed by dynamic biological systems. In biological terms, liquid water is one of the most useful substances known. Only ammonia begins to approach it in terms of providing a stable environment or flexible chemistry. Carbon can form single, double or triple bonds with other atoms and thus form a complex variety of shapes and compounds. As the most stable bond carbon forms is with other carbon atoms (hence the durability of diamond), it can create the biologically important carbon ring structures (aromatics). Carbon compounds vary from those that are highly soluble in water to those that are hydrophobic (water repellents). Water-soluble compounds are biologically reactive, and thus can easily be moved around or broken down by biological systems. Silicon is often quoted as an alternative to carbon as a basis for life, but has several drawbacks. Firstly, because silicon atoms are much bigger, they have difficulty forming double or triple bonds. This immediately restricts the variety of compounds that silicon can form. In addition, the most stable bond which silicon forms is the silicon-oxygen bond and most silicon is not soluble in water. This makes it very resistant to weathering and heating, but not biologically available. Quartz, for example, is silicon dioxide (SiO₂) - and is our most long lasting mineral, getting recycled intact when others get chemically degraded. In fact despite the fact that there is 10 times more carbon in the universe than silicon, in the Earth’s crust there is 600 times more silicon, mainly due to the fact that CO, CO₂ and CH₄ which are the most common compounds of carbon, are all gases. Most carbon bearing rocks are organic in origin (limestone, chalk etc). Silicon is used biologically - for instance in the skeletons of marine plankton such as diatoms and radiolarians - but it is not active biochemically. It may also have played a key role in the evolution of replicating molecules.

Widely differing conditions exist throughout the universe, and in the light of this, some alternative bases for chemical life have been proposed:

• Life in ammonia – liquid ammonia, perhaps with water would be a polar solvent and occur on planets with temperatures ~ -50° C. weaker chemical bonds e.g. nitrogen would predominate.

• Life in hydrocarbons – a mixture of hydrocarbons would act as the solvent, accommodating a wide range of temperatures. No polar activity, but processes involving reduction reactions would supply energy.

• Silicate life – Although limited at our temperatures and pressures, at higher temperatures (above 1000° C) the medium becomes liquid and could facilitate some evolving chemical order, although the complexity is much in doubt.

• Sulphur based life – liquid sulphur could act as the solvent and offer a rich number of metabolic pathways. We already have sulphur metabolites on Earth, though they are water based.

Some of the more extreme speculations of conditions allowing persistent self-organisation are:

• Plasma life within stars – charged particles in constraining magnetic fields.

• Life in solid hydrogen – infra red energy could be absorbed and organised by this very cold material and stored as para and ortho hydrogen molecules.

• Radiant life – based on ordered patterns of photons being absorbed and reemitted by persistent clouds of gas and dust between the stars, although these will eventually drift apart or contract.

• Life in neutron stars – physicists have proposed polymeric atoms existing on the surface of these super-compact bodies. Such arrangements could function as replicator templates, similar to DNA.

How Did Life Start?

The first attempts to experiment on the chemistry of Earth’s primordial atmosphere were made by Groth and Seuss in 1938. They exposed a CO₂/H₂O gas mixture to UV light and produced formaldehyde and glyoxal.
Miller and Urey (1953, 57) applied an electric discharge through a sealed container of methane/ammonia/hydrogen and water and synthesised amino acids such as glycine, and alanine, both important precursors of biological molecules.
Since then, numerous such "Miller experiments" have sought to trace the chemical path from primitive conditions to nucleic acid like molecules, polypeptides and fatty acids using a variety of energy sources and "recipes", both reducing and oxidising. In all of these experiments, formaldehyde and hydrogen cyanide have been vital reactants, and several robust pathways have been identified for the synthesis of biopolymers (Ponnamperuma et al 1969).
Clay minerals (alumino-silicates) have self-replication and growth properties reminiscent of life, and a high affinity for organic materials sticking to them (adsorption). It has been suggested by Kenneth Cairns-Smith and Joseph Bernal that clays may have been the templates for early life to reproduce itself, before the biological DNA/RNA replication system took over the job. The mineral known as Fool’s Gold (iron pyrites FeS₂) has been proposed by Gunter Wachtershauser as an alternative to clays in this scenario. A contrasting hypothesis is "RNA World", where RNA was the original template, without any need for an inorganic component. Primitive RNA molecules could have formed spontaneously from pre-biotic components. Once formed, this RNA had the ability to copy itself. Eventually this RNA’s ability to bind to other types of molecules (e.g. amino acids) would lead to the formation of proteins. The whole system would have become more and more complex and fine tuned over time until the detailed DNA-RNA-protein replicating mechanism we see today was created.
There is another hypothesis, first proposed in 1969, and developed in the early 80’s by Stuart Kaufmann, Manfred Eigen and Peter Shuster. It relies on writing the laws of chemistry in a much more abstract and artificial way to make it a set interaction with all the properties that we associate with reacting atoms, but non of the "carbaquist" (carbon centred) tendencies afflicting terrestrial biologists. This has led to the concept of the autocatalytic set. It seems that if you push the reaction conditions for these artificial chemistries far from equilibrium – to what is fondly known as "the edge of chaos", certain self-sustaining reaction pathways arise spontaneously. Not only are these able to persist, but they have the flexibility to mutate and evolve, becoming more complex with time. Surprisingly, many of the "molecules" produced in such environments are seeding sets that can regenerate the whole autocatalytic set again once reintroduced into the environment. Is it then written into the basic laws of physics and chemistry that complexity will arise naturally given the right conditions? If this is so, then the precursors of life might have been shaped very efficiently from the primitive broth, and the first metabolisms might have dominated the selection/competition process, long before the mutations within these sets gave rise to replicators. Such ideas have been developed extensively by Walter Fontana (he applied lambda calculus to the abstract sets he delineated – a process known as Fontana’s Alchemy) and Leo Buss to explain, not only the production of self-organising metabolisms, but the next levels on from there, namely the formation of prokaryotes and eukaryotes and even complex multicellular organisms.

Evidence of Early Life

Our evidence for early life comes in two forms:

• direct evidence, i.e. fossils of primitive lifeforms

• indirect evidence, i.e. chemical "fingerprints" of life left in the rocks

Fossil microbes are very difficult to distinguish from mineral deposits laid down by non-organic processes, so some the earliest fossils are still the subject of controversy. The oldest confirmed fossils are bacterial mats forming structures known as stromatolites. These date from 3.5 billion years ago.
The chemical signature of life is the presence of kerogen (organic material preserved in rocks) which is rich in the lightest of the carbon isotopes, ¹²C. This has been found in rocks dated 3.85 billion years old. The Earth underwent heavy bombardment from meteors in the period 4.0 to 3.8 billion years ago, which would have extinguished any life at that time. Thus life seems to have been present on Earth almost as soon as conditions permitted, if not slightly before that!
Hydrothermal vents are areas of the ocean floor where water heated by volcanic activity gushes forth at up to 360^oC. This carries with it metal and sulphur compounds from under the seabed (whose dark colour gives these vents their designation as black smokers), and these provide the energy for a whole community of organisms to survive far from the sun. Because some of the most ancient types of bacteria living today use sulphur and are very resistant to high temperatures, it has been proposed that the hydrothermal vents may have been where life first arose.

Key Evolutionary Innovations

The key events in the history of life on Earth are:

1. Evolution of oxygenic photosynthesis. This allowed an oxygen atmosphere to build up. Free oxygen may have been an essential pre-requisite for the development of eukaryotes (see below) and thus multicellular lifeforms.

1. Evolution of endo-symbiosis (primitive cells engulfing others which took up permanent residence inside them). This resulted in the eukaryotic type of cell containing various organelles (nucleus, mitochondria, chloroplasts, etc.) which divide the various tasks of the cell between them. This may be a necessary first step towards the specialisation of cells seen in multicellular organisms.

2. Evolution of multicellular body plans. Obviously, single celled organisms are small, so muticellularity had to evolve before it was possible to have large creatures.

3. Evolution of skeletal hard parts such as bones & shells. This permitted manipulation of the environment (e.g. chewing things), predation and defence, and was a pre-requisite for emergence onto land.

4. Evolution of circulatory systems. This released organisms from the necessity of obtaining oxygen by passive diffusion. They could thus increase in size beyond the half centimetre or so in thickness which is the maximum size allowed by diffusion.

5. Colonisation of the land. This had a major effect on diversity, as well as atmospheric chemistry and climate. Tool use has far greater potential on land than in the sea: fire and smelting is possible, tough organic substances like wood and bone are in abundance, rocks with naturally sharp edges are available, and if you drop something it doesn’t disappear into the depths never to be seen again…

6. Development of cleidoic (i.e. waterproof) eggs. This enabled animals to conquer dry inland areas of the continents, instead of remaining tied to wetlands and humid tropical areas.

7. Evolution of warm-bloodedness. This evolved several times (birds, mammals, dinosaurs?). It extends the range of animals into areas with permanent or seasonally cold climates, because a constant body temperature means that food availability and not environmental temperature becomes the main determination of an organism’s activity levels.

The end result of all this is a diverse fauna of large, active animals covering the surface of the Earth. The two most successful bodyplans for the conquest of the land were the vertebrates and the arthropods. The initial success of the vertebrates in the sea may have been due to their advanced immune system rather than any anatomical superiority over other marine dwellers.
Several reasons are often given for why arthropods never reach the giant size depicted in B movies. The first is that their external skeletons could not support their weight - this is not quite true. Arthropod skeletons can support a large slow moving animal easily, but if our giant insects and spiders wanted to run or jump, they would break their "bones", because exoskeletons are not good at surviving impacts. Vertebrates, on the other hand, have lots of soft tissues to cushion the impact when a foot strikes the ground. Another often stated explanation is that the arthropod form of breathing system could not supply enough oxygen to a large body. This is also unlikely, because flying insects can supply huge amounts of oxygen to their flight muscles when necessary, so a larger body doing less strenuous activity would also be possible (spiders cheat by having lung-like organs). The crucial reason, is that when creatures with an external skeleton moult, they become soft. At this stage in their life, they are in danger of getting squashed by gravity and thus damaged. Arthropods can avoid this problem by (i) being small, (ii) hanging from twigs (body is pressed against cocoon/old moult, not the ground) and (iii) going back to the water (giant fossil millipedes & eurypterids did this).

Complex Organisms

It took 2 billion years from the origin of life to the development of multicellular organisms. This is partly because of the timescale needed to build up enough oxygen in the atmosphere: it would have taken 0.75 billion years for all the inorganic sinks (mainly iron bearing rocks) to become full and thus stop soaking the oxygen up, and then a further period before the atmospheric and oceanic levels could support multicellular life. The first fossil evidence for large multicellular creatures occurs in rocks of about 650 million years old - over 3 billion years after things got started. The colonisation of the land did not start until the Silurian Period (440 million years ago), and a complete terrestrial biosphere like we have today was probably not in place until the Devonian (400–350 million years ago).
The advantage of complexity is versatility. If different regions of the body specialise for different tasks (digestion, locomotion, reproduction, etc.) then the whole organism can carry out a greater range of functions. For instance, if breathing is confined to specialised organs like gills or lungs, then the formerly delicate, permeable skin can toughen and become armoured or insulated. A cultural example of the same process is the increasing division of labour as human societies grow in size. For example in a small hunter-gatherer band, each adult needs to be able to carry out nearly all of the tasks required for survival (finding food, preparing food, teaching children, predicting weather, medical skills, etc.), but in a larger, settled community some people can specialise and trade their skills for a living (doctors, toolmakers, farmers, Continuing Education lecturers…)
However the actual process whereby complex organs evolve is still a matter of heated debate. The eye has been the traditional battleground between those arguing design and those arguing self-organisation. The C^18th theologian William Paley considered the precision engineering of the eye as proof of an intelligent creator. Even Darwin said that every time he looked at the vertebrate eye, his blood ran cold in imagining the millions of separate experiments that had honed it down the ages. Richard Dawkins offers a compelling story of the incremental development of this complex organ and backs it up with evidence culled from the fossil record (The Blind Watchmaker). In fact, it seems that there is a great selection pressure on the development of eyes since biologists are able to identify more than forty independent lineages that came up with them. These are not related to a single ancestral eye, rather they are unrelated evolutionary experiments, all ending up with a directionally sensitive light detector and imaging system.
The complexity required to produce the differentiated cells we see in a human being from some 100 000 genes coding for a different protein or enzyme seems to be so great as to require a shuffling time greater than the age of the Universe by a factor of several million. In the early 1960s a medical student by the name of William Kaufmann started to look at the properties of networks or webs. These could be interactions of simple proteins and enzymes, as found in a cell, or relations between cells themselves within an organism. By modelling these networks using light bulbs strung together, he discovered that out of all the possible ways in which the network could operate, after a few cycles it settled down to produce persistent stable patterns so called "state cycles". The speed of formation of these state cycles depended on the manner of connection of the bulbs, and there were critical numbers of connections that decided whether the system would lock in one state or veer chaotically from one state to the next. Kaufmann also found that for even a sparsely connected web of 100 000 genes there would be a handful of stable cycles out of the astronomical number of possible trajectories – interestingly enough, a number coincident with the number of different cell types in the human body (~250).
Brian Goodwin, a life scientist, believes that there are organising principles in biology that all but guarantee the formation of certain structures. He describes a "fitness landscape" in evolutionary phase space where there are deep wells of attraction that pull evolving creatures to common structures because of their superior advantage. Likewise there are thinly populated heights where only a few species reside – such as the mountain top of tool using intelligence.

Global Extinction Events

There are other key processes that have affected the evolutionary direction of life on Earth, and may well be instrumental in extraterrestrial environments.
The first of these is the natural tectonic activity of our planet’s crust. Both volcanic eruptions and the slower process of mountain building/inundation can alter the environment locally and on a global scale. Flood basalts have occurred from time to time through geological history and these massive outpourings of fluid lava and associated gases (CO₂, SO₂) can create ecological catastrophes and cause mass extinctions (90% or more of species wiped out). Supervolcanoes like the magma chamber under Yellowstone Park also have the ability to create mass extinctions, both from the initial blast wave and the subsequent ecosystem failures due to atmospheric dust.
Collisions with sizeable (1-10km) asteroids or comets have the potential for mass extinctions and it has been surmised that a combination of such events are responsible for several mass extinctions in the fossil record – the most famous being the KT boundary event of 65 million years ago – the "Dinosaur Killer". In this case, climate change favoured the homeothermic, insectivorous ancestors of all today’s mammals and allowed them to get a foothold - until then, the dinosaurs had dominated life on Earth.
If mass extinction events are too frequent either as a result of excessive vulcanism or intense bombardment, then life would have little chance to develop the complexity we see on Earth. If there were no major catastrophe, then the ecosystem might well settle into stasis. It is true that cometary bombardment would be much greater on this planet were it not for the mighty gravitational attraction of Jupiter to deflect and mop up these bodies (as witnessed in the Shoemaker-Levy 9 impact in 1994). We may well have this giant planet to thank for there being any sophisticated life on Earth.

Intelligence and Tool Use

There are trends in increasing intelligence seen in a number of animal groups on Earth (vertebrates, cephalopod molluscs, some crustaceans). Within these groups themselves, stronger trends are seen in certain lineages - for example amongst carnivores, primates and toothed whales. Increased intelligence is linked to the need to process complex information. Examples of situations like this include living in a three-dimensional environment (climbing, swimming or flying), living in groups (social skills, hierarchies, group activities), carnivory (reactions to prey behaviour - plants don’t run away or fight back!) and parental care (transmission of non-genetic information to offspring). These factors can of course act in tandem: humans and dolphins have increased intelligence due to all of these factors. All these environments and evolutionary pressures are expected to occur on other Earth-like planets, so similar trends are to be expected there.
Intelligence and tool use may operate in a feedback loop. A certain amount of intelligence is required to start tool use, but manufacture and use of tools itself provides a complex environment (anticipating the behaviour of raw materials, altering tools to fit a specific task) which may have promoted an interdependent evolution of raw intelligence and the ability to manipulate the environment. It has been suggested (e.g. Tobias 1992) that the earliest hominids (Australopithecus) had sporadic tool use not much more frequent than today’s chimpanzees, but that Homo Habilis was the first obligate tool user - it could not survive without them. Once full scale tool use was established, cultural evolution took over from biological evolution in the hominid lineage. Prior to this point, hominids adapted to changing environmental conditions by evolution in their biology or social behaviour, e.g. robust teeth to cope with a vegetarian diet, polygynous mating systems in rich environments. The addition of a cultural dimension to evolution (language, tools, fire, art) added a whole range of new ways humans could adapt to their environment - and increasingly - adapt the environment to them. To a certain extent humans are pre-adapted to tool use by having two manipulative limbs which are not needed for locomotion. Elephants’ trunks, squid tentacles and crab claws are similarly available organs, but the likes of dolphins are at a severe disadvantage. The marine environment is also rather poor in raw materials to make tools - sea plants are soft and most rocks have things growing on them. Underwater soils are soft, so organisms don’t need to use tools to dig up buried food. Plus if you live in the open ocean, then there is nothing for you to need a tool for. Smelting metals underwater is beset with problems!
Body size may be an important factor in terrestrial tool use - you need a certain amount of strength to heft a rock capable of smashing open a nut, or to use a branch as leverage (twigs are too bendy because they have less lignin in them). This can be seen in birds - a number of species fly up with things and drop them to smash them open (herring gulls, skuas, bearded vultures) but only the Egyptian vulture hammers them open with rocks whilst on the ground.

High Level Communication

The communication systems which exist on Earth include sound, vision, scent/taste, touch/vibration, electric fields, and various combinations thereof. Some senses are used primarily for communication (e.g. scent is very important in social signalling in mammals), whilst others provide effective navigation systems as well, because of the precise information about the environment they convey (vision, sonar). Communication is essential in a social animal. When the social group itself is a permanent and functional unit where all the individuals need to know each other’s status and moods, as well as acting in concert for hunting or defence (e.g. a pride of lions or troop of chimps) then detailed communication becomes even more important. It has been suggested that in early humans, gossip replaced social fur grooming as the method of reinforcing inter-group alliances and bonding. The change may have been due to the average proto-human group getting so large that everyone spent too much time grooming and not enough time eating. Technological development is only possible in a social species with overlap of generations. Solitary territorial animals are unlikely to co-operate and carry out the manufacture of even a simple device like a saucepan: someone to mine the ore, someone to smelt it, someone to shape the saucepan. And that’s without the added complications of the petrochemical industry to provide an insulated handle, non-stick coating and the gas supply to heat it on! Octopuses are very smart, but as well as being anti-social, they die just as the next generation hatches. Parents therefore never have a chance to pass on any of their experience to their offspring. Altruism may be a key factor in technological development. Humans are unusual in their willingness to interact on friendly terms with those who are not relatives or potential mates. The animal equivalent of behaviours such as helping a lost child to find their mother, buying a round of drinks for your friends or sending aid to famine victims do not exist. Altruism allowed early humans to set up trade networks and establish a system of long term favours. Projects involving the co-operation of more than one community became possible, and large settlements like modern cities are entirely dependent on the fact that we do not react adversely to strangers and have institutionalised long term favours (money, schools, hospitals).

Lifespans Large and Small

The lifespans of organisms may determine whether interstellar travel is possible. Bristlecone pines would make excellent spacefarers (they live 5000 years), however their tool-using capabilities are somewhat limited! In general, big organisms live longer than smaller ones because it takes them time to grow to their reproductive stage. For instance a generation is about 6 weeks in mice, but about 20 years in humans (because we are big animals and spend a lot of time transmitting cultural information to our offspring). It is worth remembering that a year is an arbitrary unit tied to Earth’s orbital mechanics. Planets with longer or shorter orbits and different patterns of seasons will impose different evolutionary pressures on their indigenous species. For example, plants going dormant in the winter and re-growing in the spring will occur elsewhere, but winter may not last the scant few months it does in the UK. The lifespans of species can also be looked at when thinking about colonising the galaxy. An "average" species lasts 0.5 to 1.0 million years. Long lived species clock in about 10 million years, and then there are a few living fossils that go on for hundreds of millions of years (e.g. the Pearly Nautilus). Generalist species which have a wide ecological tolerance or varied diet, tend to survive longer than specialists. They also survive catastrophes better (climate change, mass extinction). The rise of Homo Sapiens is dated at somewhere between 0.25 and 0.75 million years ago, depending on each scientist’s personal opinion on the transition point from the earlier Homo Erectus. Whether cultural and technological evolution will enable us to beat the average is a matter of conjecture!