Is there life elsewhere? Part 3.
“There is a divine purpose behind this fruitful universe.”
John writes …
I am often asked whether, as a biologist, I think there is life elsewhere in the Universe or whether I believe in aliens (the latter more typical of teenage questioners). In last month’s blog, Graham wrote about this question from the point of view of a physicist who is also intrigued by biology. I am now adding my views on that question. One would think that I could simply answer ‘Yes’ or ‘No’ but I cannot. As I weigh up the possibilities, I cannot reach a firm conclusion – sometimes I think ‘Yes’, sometimes I think ‘No’ but mostly I think ‘I just do not know’. It is a question about which I am truly agnostic.
The first point to consider is the definition of life and that turns out to be harder than we might suppose. In a recent panel discussion on Radio Maria (Science and Faith: Episode 6 – The Biology of Human Life – Radio Maria England) we agreed that it is easy enough to describe life in terms of the attributes and activities that we ascribe to living things. Some of our readers may be familiar with the acronym Mrs Gren which helps us to remember the essential activities of living organisms (Movement, including movement within cells; Respiration, meaning the ‘extraction’ of energy by breakdown of particular substrates; Sensitivity – response to environment etc.; Growth; Reproduction; Excretion; Nutrition). So according to this, a living thing is defined as living because it carries out these activities (but not necessarily all the time). However, in our panel discussion we found it impossible to come up with a stand-alone definition of life – any definition that we came up with was effectively dependent on the descriptors/activities mentioned above. Nevertheless, we agreed that we can tell living things apart from non-living things such a rocks and water, although viruses defy a characterisation as either living or non-living.
The second point is whether life elsewhere in the Universe would be similar to life on Earth or different from it. The problem here is that we cannot envisage any other system of chemistry that might lead to the development of entities that exhibit the ‘Mrs Gren’ characteristics mentioned above. Further, based on the uniformity of the laws of physics, it seems very unlikely that there exist any locations in the Universe where an entirely different system of potentially life-supporting chemistry exists. That leads me to suggest that if there is life on other planets it will be based on the chemistry of carbon whose structure and reactivity is ideal for formation of a wide range of complex biochemicals. Having said that, we need to think about whether the use of carbon would result in similar biochemistry in a distant alien life-form to that which pertains on Earth.
This leads on to my third point, namely what are the essential basic features of chemistry/biochemistry that would enable life to start? As I describe in Chapter 5, many biologists regard the existence of a ‘self-replicating’ molecule (i.e., a molecule that can be copied and passed on to succeeding generations) as an essential pre-requisite for life. However, building larger molecules from smaller molecules requires an input of energy and, as also described in Chapter 5, Nick lane and other biochemists suggest that an ‘energy-gathering’ system (loosely defined as ‘respiration’ in the Mrs Gren acronym) is also an essential pre-requisite. Thus, we have two essentials that must be in place together before there is life. Intriguingly, these two essentials are manifest in the same ways in all living organisms. All possess DNA as their ‘self-replicating’ molecule and, at the heart of all energy-gathering systems there lies the process of creating a gradient of protons (positively charged sub-atomic particles). In seeing these two features we believe that we are getting a glimpse of what the earliest life-forms might have been like.
Fourthly, there is a bit more to the simplest living organism than just the two essentials. In order to prevent dilution by and loss to the surrounding medium, there needs to be containment, That is, a cell with a boundary between the inside and the outside. Further, there must be information to ‘run’ the cell. This is provided by the ‘code’ in DNA (which is the same in all organisms) and I have written in Chapter 5 of the improbability of the decoding mechanisms occurring spontaneously.
My fifth and final point is the question of whether there are other planets capable of supporting life. We noted in the book that Earth is a very special place with many features that help life to flourish. How likely is it that, amongst the vast numbers of planetary systems that Graham mentioned in our March blog post, there exists ‘another Earth’ or even several other Earths? On the basis of those vast numbers we might suggest intuitively that such planets must exist but actually we have no real means of calculating the probability. And if there is indeed another Earth somewhere, how likely is it that life exists there? I have already pointed out that, even on Earth, the origin of life (abiogenesis) seems improbable and yet it occurred relatively soon in the evolution of our planet. Does that mean that it will occur wherever and whenever planetary conditions permit?
I leave you with a thought expressed several times by the late John Polkinghorne, namely that God has created a finely tuned and fruitful universe (For example, Introduction (wordpress.com)). If God the creator has willed it, life is possible on any other suitable planet. But of course we do not know ‘the mind of God’ in relation to our question.
John Bryant and Graham Swinerd comment on biology, physics and faith.