Life in the Universe
In this talk, I would like to speculate a little, on the development of life in
the universe, and in particular, the development of intelligent life. I shall take this to
include the human race, even though much of its behavior through out history, has been
pretty stupid, and not calculated to aid the survival of the species. Two questions
I shall discuss are: what is the probability of life existing else where in the universe.
And how may life develop in the future.
It is a matter of common experience, that things get more disordered and chaotic with
time. This observation can be elevated to the status of a law, the so-called Second Law of
Thermodynamics. This says that the total amount of disorder, or entropy, in the universe,
always increases with time. However, the Law refers only to the ~total amount of disorder.
The order in one body can increase, provided that the amount of disorder in its
surroundings, increases by a greater amount. This is what happens in a living being. One
can define Life to be an ordered system, that can sustain itself against the tendency to
disorder, and can reproduce itself. That is, it can make similar, but independent, ordered
systems. To do these things, the system must convert energy in some ordered form, like
food, sunlight, or electric power, into disordered energy, in the form of heat. In this
way, the system can satisfy the requirement, that the ~total amount of disorder increases,
while at the same time, increasing the order in itself, and its offspring.
A living being usually has two elements: a set of instructions, that tell the system how
to sustain and reproduce itself. And a mechanism to carry out the instructions. In
biology, these two parts are called, genes, and metabolism. But it is worth emphasizing,
that there need be nothing biological about them. For example, a computer virus is a
program that will make copies of itself in the memory of a computer, and will transfer
itself to other computers. Thus it fits the definition of a living system, that I have
given. Like a biological virus, it is a rather degenerate form, because it contains only
instructions or genes, and doesn’t have any metabolism of its own. Instead, it
reprograms the metabolism of the host computer, or cell. Some people have questioned
whether viruses should count as life, because they are parasites, and can not exist
independently of their hosts. But then most forms of life, ourselves included, are
parasites, in that they feed off, and depend for their survival, on other forms of life. I
think computer viruses should count as life. Maybe it says something about human nature,
that the only form of life we have created so far, is purely destructive. Talk about
creating life in our own image. I shall return to electronic forms of life later on.
What we normally think of as life, is based on chains of carbon atoms, with a few other
atoms, such as nitrogen or phosphorous. One can speculate that one might have life
with some other chemical basis, such as silicon, but carbon seems the most favorable case,
because it has the richest chemistry. That carbon atoms should exist at all, with the
properties that they have, requires a fine adjustment of physical constants, such as the
QCD scale, the electric charge, and even the dimension of spacetime. If these constants
had significantly different values, either the nucleus of the carbon atom would not be
stable, or the electrons would collapse in on the nucleus. At first sight, it seems
remarkable that the universe is so finely tuned. Maybe this is evidence, that the universe
was specially designed to produce the human race. However, one has to be careful about
such arguments, because of what is known as the Anthropic Principle. This is based on the
self evident truth, that if the universe had not been suitable for life, we wouldn’t
be asking why it is so finely adjusted. One can apply the Anthropic Principle, in
either its Strong, or Weak, versions. For the Strong Anthropic Principle, one
supposes that there are many different universes, each with different values of the
physical constants. In a small number, the values will allow the existence of objects like
carbon atoms, that can act as the building blocks of living systems. Since we must live in
one of these universes, we should not be surprised that the physical constants are finely
tuned. If they weren’t, we wouldn’t be here.
The strong form of the anthropic principle, is not very satisfactory. What operational
meaning, can one give to the existence of all those other universes. And if they are
separate from our own universe, how can what happens in them, affect our universe.
Instead, I shall adopt what is known as the Weak Anthropic Principle. That is, I shall
take the values of the physical constants, as given. But I shall see what
conclusions can be drawn, from the fact that life exists on this planet, at this stage in
the history of the universe.
There was no carbon, when the universe began in the Big Bang, about 15 billion years ago.
It was so hot, that all the matter would have been in the form of particles, called
protons and neutrons. There would initially have been equal numbers of protons and
neutrons. However, as the universe expanded, it would have cooled. About a minute
after the Big Bang, the temperature would have fallen to about a billion degrees, about a
hundred times the temperature in the Sun . At this temperature, the neutrons will start to
decay into more protons. If this had been all that happened, all the matter in the
universe would have ended up as the simplest element, hydrogen, whose nucleus consists of
a single proton. However, some of the neutrons collided with protons, and stuck together
to form the next simplest element, helium, whose nucleus consists of two protons and two
neutrons. But no heavier elements, like carbon or oxygen, would have been formed in the
early universe. It is difficult to imagine that one could build a living system, out of
just hydrogen and helium, and anyway the early universe was still far too hot for atoms to
combine into molecules.
The universe would have continued to expand, and cool. But some regions would have had
slightly higher densities, than others. The gravitational attraction of the extra matter
in those regions, would slow down their expansion, and eventually stop it. Instead, they
would collapse to form galaxies and stars, starting from about two billion years after the
Big Bang. Some of the early stars would have been more massive than our Sun . They
would have been hotter than the Sun, and would have burnt the original hydrogen and
helium, into heavier elements, such as carbon, oxygen, and iron. This could have taken
only a few hundred million years. After that, some of the stars would have exploded as
supernovas, and scattered the heavy elements back into space, to form the raw
material for later generations of stars.
Other stars are too far away, for us to be able to see directly, if they have
planets going round them. But certain stars, called pulsars, give off regular pulses of
radio waves. We observe a slight variation in the rate of some pulsars, and this is
interpreted as indicating that they are being disturbed, by having Earth sized
planets going round them. Planets going round pulsars are unlikely to have life, because
any living beings would have been killed, in the supernova explosion that led to the star
becoming a pulsar. But the fact that several pulsars, are observed to have planets,
suggests that a reasonable fraction of the hundred billion stars in our galaxy, may also
have planets. The necessary planetary conditions for our form of life, may therefore have
existed from about four billion years after the Big Bang.
Our solar system was formed about four and a half billion years ago, or about ten
billion years after the Big Bang, from gas contaminated with the remains of earlier stars.
The Earth was formed largely out of the heavier elements, including carbon and oxygen.
Somehow, some of these atoms came to be arranged in the form of molecules of DNA. This has
the famous double helix form, discovered by Crick and Watson, in a hut on the New Museum
site in Cambridge. Linking the two chains in the helix, are pairs of nucleic acids. There
are four types of nucleic acid, aden-nine, cyto-sine, guan-nin, and thy-mine . I’m
afraid my speech synthesizer is not very good, at pronouncing their names. Obviously, it
was not designed for molecular biologists. An aden-nine on one chain, is always
matched with a thy-mine on the other chain, and a guanine with a cyto-sine. Thus the
sequence of nucleic acids on one chain, defines a unique complementary sequence, on the
other chain. The two chains can then separate, and each act as templates to build further
chains. Thus DNA molecules can reproduce the genetic information, coded in their sequences
of nucleic acids. Sections of the sequence, can also be used to make proteins and other
chemicals, that can carry out the instructions coded in the sequence, and assemble the raw
material for DNA to reproduce itself.
We do not know how DNA molecules first appeared. The chances against a DNA molecule
arising by random fluctuations, are very small. Some people have therefore suggested, that
life came to Earth from elsewhere, and that there are seeds of life floating round in the
galaxy. However, it seems unlikely that DNA could survive for long in the radiation in
space. And even if it could, it would not really help explain the origin of life, because
the time available since the formation of carbon, is only just over double the age of the
Earth.
One possibility is, that the formation of something like DNA, that could reproduce itself,
really is extremely unlikely. However, in a universe with a very large, or
infinite number of stars, one would expect it to occur in a few stellar systems, but they
would be very widely separated. The fact that life happened to occur on Earth, is not
however surprising or unlikely. It is just an application of the Weak Anthropic
Principle: if life had appeared instead on another planet, we would be asking why it had
occured there.
If the appearance of life on a given planet, was very unlikely, one might have
expected it to take a long time. More precisely, one might have expected life to appear,
just in time for the subsequent evolution, to intelligent beings like us, to have occured
before the cut off, provided by the life time of the Sun . This is about ten billion
years, after which the Sun will swell up and engulf the Earth. An intelligent form of
life, might have mastered space travel, and be able to escape to another star. But
otherwise, life on Earth would be doomed.
There is fossil evidence, that there was some form of life on Earth, about three and a
half billion years ago. This may have been only 500 million years after the Earth became
stable and cool enough, for life to develop. But life could have taken 7 billion years to
develop, and still have left time to evolve to beings like us, who could ask about the
origin of life. If the probability of life developping on a given planet, is very small,
why did it happen on Earth, in about one 14th of the time available.
The early appearance of life on Earth, suggests that there’s a good chance of the
spontaneous generation of life, in suitable conditions. Maybe there was some simpler form
of organisation, which built up DNA. Once DNA appeared, it would have been so successful,
that it might have completely replaced the earlier forms. We don’t know what these
earlier forms would have been. One possibility is RNA. This is like DNA, but rather
simpler, and without the double helix structure. Short lengths of RNA, could reproduce
themselves like DNA, and might eventually build up to DNA. One can not make nucleic acids
in the laboratory, from non living material, let alone RNA. But given 500 million years,
and oceans covering most of the Earth, there might be a reasonable probability of RNA,
being made by chance.
As DNA reproduced itself, there would have been random errors. Many of these errors would
have been harmful, and would have died out. Some would have been neutral, that is they
would not have affected the function of the gene. Such errors would contribute to a
gradual genetic drift, that seems to occur in all populations. And a few errors would have
been favorable to the survival of the species. These would have been chosen by Darwinian
natural selection.
The process of biological evolution was very slow at first. It took two and a half billion
years, to evolve from the earliest cells, to multi cell animals, and another billion years
to evolve through fish and reptiles, to mammals. But then evolution seemed to have
speeded up. It only took about a hundred million years, to develop from the early mammals
to us. The reason is, fish contain most of the important human organs, and mammals,
essentially all of them. All that was required to evolve from early mammals, like lemurs,
to humans, was a bit of fine tuning.
But with the human race, evolution reached a critical stage, comparable in importance with
the development of DNA. This was the development of language, and particularly written
language. It meant that information can be passed on, from generation to generation, other
than genetically, through DNA. There has been no detectable change in human DNA, brought
about by biological evolution, in the ten thousand years of recorded history. But the
amount of knowledge handed on from generation to generation, has grown enormously. The DNA
in human beings, contains about three billion nucleic acids. However, much of the
information coded in this sequence, is redundant, or is inactive. So the total amount of
useful information in our genes, is probably something like a hundred million bits. One
bit of information is the answer to a yes no question. By contrast, a paper back
novel might contain two million bits of information. So a human is equivalent to 50 Mills
and Boon romances. A major national library can contain about five million books, or about
ten trillion bits. So the amount of information handed down in books, is a hundred
thousand times as much as in DNA.
Even more important, is the fact that the information in books, can be changed, and
updated, much more rapidly. It has taken us several million years to evolve from the apes.
During that time, the useful information in our DNA, has probably changed by only a few
million bits. So the rate of biological evolution in humans, is about a bit a year. By
contrast, there are about 50,000 new books published in the English language each year,
containing of the order of a hundred billion bits of information. Of course, the great
majority of this information is garbage, and no use to any form of life. But, even so, the
rate at which useful information can be added, is millions, if not billions, higher than
with DNA.
This has meant that we have entered a new phase of evolution. At first, evolution
proceeded by natural selection, from random mutations. This Darwinian phase, lasted about
three and a half billion years, and produced us, beings who developed language, to
exchange information. But in the last ten thousand years or so, we have been in what might
be called, an external transmission phase. In this, the ~internal record of information,
handed down to succeeding generations in DNA, has not changed significantly. But the
~external record, in books, and other long lasting forms of storage, has grown enormously.
Some people would use the term, evolution, only for the internally transmitted genetic
material, and would object to it being applied to information handed down externally. But
I think that is too narrow a view. We are more than just our genes. We may be no stronger,
or inherently more intelligent, than our cave man ancestors. But what distinguishes us
from them, is the knowledge that we have accumulated over the last ten thousand years, and
particularly, over the last three hundred. I think it is legitimate to take a broader
view, and include externally transmitted information, as well as DNA, in the evolution of
the human race.
The time scale for evolution, in the external transmission period, is the time scale for
accumulation of information. This used to be hundreds, or even thousands of years. But now
this time scale has shrunk to about 50 years, or less. On the other hand, the brains with
which we process this information, have evolved only on the Darwinian time scale, of
hundreds of thousands of years. This is beginning to cause problems. In the 18th
century, there was said to be a man who had red every book written. But nowadays, if you
read one book a day, it would take you about 15,000 years to read through the books in a
national Library. By which time, many more books would have been written.
This has meant that no one person, can be the master of more than a small corner of human
knowledge. People have to specialize, in narrower and narrower fields. This is
likely to be a major limitation in the future. We certainly can not continue for long,
with the exponential rate of growth of knowledge, that we have had in the last three
hundred years. An even greater limitation and danger for future generations, is that we
still have the instincts, and in particular, the aggressive impulses, that we had in cave
man days. Aggression, in the form of subjugating or killing other men, and taking their
women and food, has had definite survival advantage, up to the present time. But now it
could destroy the entire human race, and much of the rest of life on Earth. A nuclear war,
is still the most immediate danger, but there are others, such as the release of a
genetically engineered virus. Or the green house effect becoming unstable.
There is no time, to wait for Darwinian evolution, to make us more intelligent, and better
natured. But we are now entering a new phase, of what might be called, self designed
evolution, in which we will be able to change and improve our DNA. There is a project now
on, to map the entire sequence of human DNA. It will cost a few billion dollars, but that
is chicken feed, for a project of this importance. Once we have red the book of life, we
will start writing in corrections. At first, these changes will be confined to the repair
of genetic defects, like cystic fibrosis, and muscular dystrophy. These are controlled by
single genes, and so are fairly easy to identify, and correct. Other qualities, such as
intelligence, are probably controlled by a large number of genes. It will be much more
difficult to find them, and work out the relations between them. Nevertheless, I am sure
that during the next century, people will discover how to modify both intelligence, and
instincts like aggression.
Laws will be passed, against genetic engineering with humans. But some people won’t be
able to resist the temptation, to improve human characteristics, such as size of memory,
resistance to disease, and length of life. Once such super humans appear, there are
going to be major political problems, with the unimproved humans, who won’t be
able to compete. Presumably, they will die out, or become unimportant. Instead, there will
be a race of self designing beings, who are improving themselves at an ever increasing
rate.
If this race manages to redesign itself, to reduce or eliminate the risk of self
destruction, it will probably spread out, and colonize other planets and stars.
However, long distance space travel, will be difficult for chemically based life
forms, like DNA. The natural life time for such beings, is short, compared to the travel
time. According to the theory of relativity, nothing can travel faster than light. So the
round trip to the nearest star would take at least 8 years, and to the center of the
galaxy, about a hundred thousand years. In science fiction, they overcome this
difficulty, by space warps, or travel through extra dimensions. But I don’t think these
will ever be possible, no matter how intelligent life becomes. In the theory of
relativity, if one can travel faster than light, one can also travel back in time. This
would lead to problems with people going back, and changing the past. One would also
expect to have seen large numbers of tourists from the future, curious to look at our
quaint, old fashioned ways.
It might be possible to use genetic engineering, to make DNA based life survive
indefinitely, or at least for a hundred thousand years. But an easier way, which is almost
within our capabilities already, would be to send machines. These could be designed
to last long enough for interstellar travel. When they arrived at a new star, they could
land on a suitable planet, and mine material to produce more machines, that could be sent
on to yet more stars. These machines would be a new form of life, based on mechanical and
electronic components, rather than macro-molecules. They could eventually replace DNA
based life, just as DNA may have replaced an earlier form of life.
This mechanical life could also be self designing. Thus it seems that the external
transmission period of evolution, will have been just a very short interlude, between the
Darwinian phase, and a biological, or mechanical, self design phase. This is shown on this
next diagram, which is not to scale, because there’s no way one can show a period of ten
thousand years, on the same scale as billions of years. How long the self design phase
will last, is open to question. It may be unstable, and life may destroy itself, or get
into a dead end. If it does not, it should be able to survive the death of the Sun, in
about 5 billion years, by moving to planets around other stars. Most stars will have burnt
out in another 15 billion years or so, and the universe will be approaching a state of
complete disorder, according to the Second Law of Thermodynamics. But Freeman Dy-son has
shown that, despite this, life could adapt to the ever decreasing supply of ordered
energy, and therefore could in principle continue forever.
What are the chances that we will encounter some alien form of life, as we explore the
galaxy. If the argument about the time scale for the appearance of life on Earth, is
correct, there ought to be many other stars, whose planets have life on them. Some of
these stellar systems, could have formed 5 billion years before the Earth. So why is
the galaxy not crawling with self designing mechanical or biological life forms. Why hasn’t
the Earth been visited, and even colonized. I discount suggestions that UFO’s contain
beings from outer space. I think any visits by aliens, would be much more obvious, and
probably also, much more unpleasant.
What is the explanation, of why we have not been visited. One possibility is, that the
argument about the appearance of life on Earth, is wrong. Maybe the probability of life
spontaneously appearing is so low, that Earth is the only planet in the galaxy, or in the
observable universe, in which it happened. Another possibility is that there was a
reasonable probability of forming self reproducing systems, like cells, but that most of
these forms of life did not evolve intelligence. We are used to thinking of intelligent
life, as an inevitable consequence of evolution. But the Anthropic Principle, should warn
us to be beware of such arguments. It is more likely that evolution is a random process,
with intelligennce, as only one of a large number of possible outcomes. It is not
clear that intelligence has any long term survival value. Bacteria, and other single cell
organisms, will live on, if all other life on Earth is wiped out by our actions. There is
support for the view that intelligence, was an unlikely development for life on Earth,
from the chronology of evolution. It took a very long time, two and a half billion years,
to go from single cells, to multi cell beings, who are a necessary precursor to
intelligence. This is a good fraction of the total time available, before the Sun blows
up. So it would be consistent with the hypothesis, that the probability for life to
develop intelligence, is low. In this case, we might expect to find many other life forms
in the galaxy, but we are unlikely to find intelligent life. Another way in which life
could fail to develop to an intelligent stage, would be if an asteroid or comet were to
collide with the planet. We have just observed the collision of the comet,
Schumacher-Levi, with Jupiter. It produced a series of enormous fire balls. It is thought
the collision of a rather smaller body with the Earth, about 70 million years ago, was
responsible for the extinction of the dinosaurs. A few small early mammals survived, but
anything as large as a human, would have almost certainly been wiped out. It is difficult
to say how often such collision occur, but a reasonable guess might be, every twenty
million years, on average. If this figure is correct, it would mean that intelligent life
on Earth, developed only because of the lucky chance, that there have been no major
collisions in the last 70 million years. Other planets in the galaxy on which life has
developed, may not have had a long enough collision free period, to evolve intelligent
beings.
A third possibility, is that there is a reasonable probability for life to form, and to
evolve to intelligent beings in the external transmission phase. But at that point, the
system becomes unstable, and the intelligent life destroys itself. This would be a very
pessimistic conclusion. I very much hope it isn’t true. I prefer a fourth possibility:
there are other forms of intelligent life out there, but that we have been overlooked.
There used to be a project called Setti, the search for extra-terestial
intelligence. It involved scanning the radio frequencies, to see if we could pick up
signals from alien civilizations. I thought this project was worth supporting,
though it was canceled for lack of funds. But we should have been wary of answering back,
until we have develop a bit further. Meeting a more advanced civilization, at
our present stage, might be a bit like the original inhabitants of America, meeting
Columbus. I don’t think they were better off for it.
That is all I have to say. Thank you for listenning.
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