Stephen Hawking discusses DNA,the origin of life, genetic engineering, self-designed evolution, and possible future paths of human evolution:
LIFE IN THE UNIVERSE
by Stephen Hawking (from Scientific American)
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 behaviour throughout 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, life 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 supernovae 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:
adenine, cytosine, guanine and thymine. An adenine on one chain is always
matched with a thymine on the other chain, and a guanine with a cytosine.
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 fantastically 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 occurred 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
occurred before the cutoff 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 developing
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 paperback
novel might contain two million bits of information. So a human is equivalent
to
50 Mills and Boon romances. The University Library contains 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, times 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
read
every book written. But nowadays if you read one book a day it would take
you
about 15000 years to read through the books in the University 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 read 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
in-definitely 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 onto 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 Dyson 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 intelligence
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.
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 is a project now on called SETI the search for extraterrestrial
intelligence. It involves scanning the radio frequencies to see if we can
pick up
signals from alien civilizations. I think this project is worth supporting.
But we
should be wary of answering back. Meeting a more advanced civilization
might be
a bit like the original inhabitants of America meeting Columbus. I don't
think they
were better off for it.
