Astrobiology » Outward Appearances

In considering the possibility of life on other worlds we must ask two important questions – outward appearances and biochemical differences. We now consider the outward appearances.

Life on Earth is usually divided into a number of major groups or 'kingdoms'. The simplest major divisions are Viruses. Single Cell Microorganisms and Multicellular organisms. Viruses are submicroscopic agents that infect living organisms, often causing disease, and that consist of a single or double strand of RNA or DNA surrounded by a protein coat. They are unable to replicate without a host cell and many scientists do not class viruses as living organisms. Microorganisms include archaea, bacteria, protozoa and single celled plants. There are some primitive organisms which thrive in the absence of oxygen some of which are in fact poisoned by oxygen. They are referred to as anaerobic microorganisms. Other simple microorganisms require oxygen – they are referred to as aerobic. There is one major class of bacteria of paramount importance – they are a class known as CYANOBACTERIA. They have evolved photosynthesise and give out oxygen in the presence of light. They are extremely important in the evolution of life on Earth and were responsible for changing the atmosphere of the early Earth and eventually making the evolution of plants, animals and fungi possible.

At some point in the early history of life single celled organisms underwent an important change and collaborated together to form multicellular organisms. The main subdivisions of multicellular organisms are PLANTS, FUNGI AND ANIMALS. Like cyanobacteria, plants photosynthesise and give off oxygen in the presence of light. Animals and fungi do not photosynthesise. Fungi and animals are dependant on plants because they require oxygen in order to live.

Even if life forms relied on a different biochemistry to that on Earth, they could well outwardly resemble many familiar forms of life on Earth. There is an important phenomenon in biology known as BIOLOGICAL CONVERGENCE. In evolutionary biology, convergent evolution is the process whereby organisms that are not closely related evolve similar traits as a result of having to adapt to similar environments. They occupy similar ecological niches. For example THE ABILITY TO FLY evolved separately in quite different types of animals insects, pterodactyls, birds and bats. Another example is the eye. The EYE evolved separately in the octopus and squid group (cephalopods) and in the vertebrates. It would be hard to find two such different groups of animals as vertebrates and cephalopods. The implications for alien life forms are very great. Even if they used a totally different biochemistry they might still resemble life on Earth.

Ray Goodwin

Somewhere there are mountains
Glistening in the snow
Somewhere there are mountains
That we shall never know

Somewhere there are rivers
Flowing fast and free
Somewhere there are rivers
That we can never see

Somewhere there are oceans
And sun drenched island sands
Forests full of creatures
In vastly distant lands

Somewhere there’s a planet
Beneath an alien star
The people watch our tiny sun
And wonder where we are

One day perhaps we’ll find them
Across the void of space
Perhaps through ways as yet unknown
We’ll meet them face to face

The author of this web site Ray Goodwin holds B.Sc. Degrees from London University in Chemistry, Geology and Physiology and an M.Sc. in Biochemistry. He has spent most of his professional life teaching in Colleges of Technology. On his retirement he has entered the fields of astronomy, astrochemistry, astrobiology and space sciences. He has spent a great deal of his retirement in visiting amateur astronomy societies and in attending European Space Agency Symposia in ESTEC in the Netherlands and other scientific conferences in England and Sweden. He regularly attends the yearly European Astrofest in South Kensington London and other meetings in the UK. He has written scientific articles and given a number of lectures on diverse scientific subjects.

Readers of this web site are invited to e-mail the author ( and discuss their opinions of the topics dealt with and suggest any changes which they think may be helpful.

Life in the Cosmos Website
Version 01.00 - April 20, 2015.