The theory of evolution is one of the best-known
scientific theories around. Try to make it through a day
without using or hearing the word "evolution" and you'll see
just how widespread this theory is.
Evolution is fascinating because it attempts to answer one
of the most basic human questions: Where did life, and human
beings, come from? The theory of evolution proposes that life
and humans arose through a natural process. A very large
number of people do not believe this, which is something that
keeps evolution in the news.
In this edition of HowStuffWorks,
we will explore the theory of evolution and how it works. We
will also examine several important areas that show holes in
the current theory -- places where scientific research will be
working in the coming years in order to complete the theory.
The holes are considered by many to be proof that the theory
of evolution should be overthrown. As a result, quite a bit of
controversy has surrounded evolution ever since it was first
Let's start off by taking a look at the basic principles of
the theory of evolution, look at some examples and then
examine the holes.
The Basic Process of Evolution
theory of evolution is surprisingly simple. It has three
- It is possible for the DNA of an
organism to occasionally change, or mutate. A mutation
changes the DNA of an organism in a way that affects its
offspring, either immediately or several generations down
- The change brought about by a mutation is either
beneficial, harmful or neutral. If the change is
harmful, then it is unlikely that the offspring will survive
to reproduce, so the mutation dies out and goes nowhere. If
the change is beneficial, then it is likely that the
offspring will do better than other offspring and so will
reproduce more. Through reproduction, the beneficial
mutation spreads. The process of culling bad mutations and
spreading good mutations is called natural selection.
- As mutations occur and spread over long periods of
time, they cause new species to form. Over the course of
many millions of years, the processes of mutation and
natural selection have created every species of life that we
see in the world today, from the simplest bacteria to humans
and everything in between.
of years ago, according to the theory of evolution, chemicals
randomly organized themselves into a self-replicating
molecule. This spark of life was the seed of every living
thing we see today (as well as those we no longer see, like
dinosaurs). That simplest life form, through the processes of
mutation and natural selection, has been shaped into every
living species on the planet.
In the book "The
Dragons of Eden," Carl Sagan summarized the theory of
evolution in this way:
useful mutations provide the working material for biological
evolution -- as, for example, a mutation for melanin in
certain moths, which changes their color from white to
black. Such moths commonly rest on English birch trees,
where their white coloration provides protective camouflage.
Under these conditions, the melanin mutation is not an
advantage -- the dark moths are starkly visible and are
eaten by birds; the mutation is selected against. But when
the Industrial Revolution began to cover the birch bark with
soot, the situation was reversed, and only moths with the
melanin mutation survived. Then the mutation was selected
for, and, in time, almost all of the moths are dark, passing
this inheritable change on to future generations. There are
still occasional reverse mutations eliminating the melanin
adaptation, which would be useful for the moths were English
industrial pollution to be controlled. Note that in all this
interaction between mutation and natural selection, no moth
is making a conscious effort to adapt to a changed
environment. The process is random and statistical.
Can such a simple theory explain all of life as we know it
today? Let's start by understanding how life works and then
look at some examples.
How Life Works: DNA and Enzymes
can be seen in its purest form in the daily evolution of
bacteria. If you have read How Cells
Work, then you are familiar with the inner workings of the
E. coli bacteria and can skip this
section. Here's a quick summary to highlight the most
important points in How Cells
- A bacterium is a small, single-celled organism. In the
case of E. coli, the bacteria are about one-hundredth the
size of a typical human cell. You can think of the bacteria
as a cell wall (think of the cell wall as a tiny
plastic bag) filled with various proteins, enzymes and other
molecules, plus a long strand of DNA, all floating
- The DNA strand in E. coli contains about 4 million base
pairs, and these base pairs are organized into about
1,000 genes. A gene is simply a template for a
protein, and often these proteins are enzymes.
- An enzyme
is a protein that speeds up a particular chemical
reaction. For example, one of the 1,000 enzymes in an E.
coli's DNA might know how to break a maltose molecule (a
simple sugar) into its two glucose molecules. That is all
that that particular enzyme can do, but that action is
important when an E. coli is eating maltose. Once the
maltose is broken into glucose, other enzymes act on the
glucose molecules to turn them into energy for the cell to
- To make an enzyme that it needs, the chemical mechanisms
inside an E. coli cell make a copy of a gene from the
DNA strand and use this template to form the enzyme.
The E. coli might have thousands of copies of some enzymes
floating around inside it, and only a few copies of others.
The collection of 1,000 or so different types of enzymes
floating in the cell makes all of the cell's chemistry
possible. This chemistry makes the cell "alive" -- it allows
the E. coli to sense food, move
around, eat and reproduce. See How Cells
Work for more details.
You can see that, in any living cell, DNA helps create
enzymes, and enzymes create the chemical reactions that are
Bacteria reproduce asexually.
This means that, when a bacteria cell splits, both halves of
the split are identical -- they contain exactly the same DNA.
The offspring is a clone of
As explained in How
Human Reproduction Works, higher organisms like plants,
insects and animals reproduce sexually, and this
process makes the actions of evolution more interesting.
Sexual reproduction can create a tremendous amount of
variation within a species. For example, if two parents have
multiple children, all of the children can be remarkably
different. Two brothers can have different hair color,
different heights, different blood types
and so on. Here's why that happens:
Because of the random
nature of gene selection, each child gets a different mix of
genes from the DNA of the mother and father. This is why
children from the same parents can have so many differences.
- Instead of a long loop of DNA like a bacterium, cells of
plants and animals have chromosomes that hold the DNA
strands. Humans have 23 pairs of chromosomes, for a total of
46 chromosomes. Fruit flies have five pairs. Dogs have 39
pairs, and some plants have as many as 100.
The human chromosomes hold the DNA of
the human genome. Each parent contributes 23
- Chromosomes come in pairs. Each chromosome is a tightly
packed strand of DNA. There are two strands of DNA
joined together at the centromere to form an X-shaped
structure. One strand comes from the mother and one from the
- Because there are two strands of DNA, it means that
animals have two copies of every gene, rather than
one copy as in an E. coli cell.
Photo courtesy U.S. DOE, Human Genome
- When a female creates an egg or a male creates a sperm,
the two strands of DNA must combine into a single
strand. The sperm and egg from the mother and father
each contribute one copy of each chromosome. They meet to
give the new child two copies of each gene.
- To form the single strand in the sperm or egg, one or
the other copy of each gene is randomly chosen. One
or the other gene from the pair of genes in each chromosome
gets passed on to the child.
A gene is nothing but a template for creating an enzyme.
This means that, in any plant or animal, there are actually
two templates for every enzyme. In some cases, the two
templates are the same (homozygous), but in many cases
the two templates are different (heterozygous).
Here is a well-known example from pea plants that helps
understand how pairs of genes can interact. Peas can be tall
or short. The difference comes, according to Carol Deppe in
the book "Breed
your own Vegetable Varieties":
synthesis of a plant hormone called gibberellin. The "tall"
version of the gene is normally the form that is found in
the wild. The "short" version, in many cases, has a less
active form of one of the enzymes involved in the synthesis
of the hormone, so the plants are shorter. We refer to two
genes as alleles of each other when they are
inherited as alternatives to each other. In molecular terms,
alleles are different forms of the same gene. There
can be more than two alleles of a gene in a population of
organisms. But any given organism has only two alleles at
the most. Shorter plants usually cannot compete with the
taller forms in the wild. A short mutant in a patch of tall
plants would be shaded out. That problem isn't relevant when
a human plants a patch or field with nothing but short
plants. And short plants may be earlier than tall ones, or
less subject to lodging (falling over) in the rain or wind.
They also may have a higher proportion of grain to the rest
of the plant. So shorter plants can be advantageous as
cultivated crops. Specific mutations or alleles are not good
or bad in and of themselves, but only within a certain
context. An allele that promotes better growth in hot
weather may promote inferior growth in cold weather, for
One thing to notice in Deppe's quote is that a mutation in
a single gene may have no effect on an organism, or its
offspring, or its offspring's offspring. For example, imagine
an animal that has two identical copies of a gene in one allele.
A mutation changes one of the two genes in a harmful way.
Assume that a child receives this mutant gene from the father.
The mother contributes a normal gene, so it may have no effect
on the child (as in the case of the "short" pea gene). The
mutant gene might persist through many generations and never
be noticed until, at some point, both parents of a child
contribute a copy of the mutant gene. At that point, taking
the example from Deppe's quote, you might get a short pea
plant because the plant does not form the normal amount of
Another thing to notice is that many different forms of a
gene can be floating around in a species. The combination of
all of the versions of all of the genes in a species is called
pool of the species. The gene pool increases when a
mutation changes a gene and the mutation survives. The gene
pool decreases when a gene dies out.
One of the simplest examples of evolution can be witnessed
in an E. coli cell. To get a better grip on the process, we'll
take a look at what happens in this cell.
The Simplest Example of Evolution
process of evolution acts on an E. coli cell by creating a
mutation in the DNA. It is not uncommon for the DNA strand in
an E. coli bacterium to get corrupted. An X-ray, a cosmic ray
or a stray chemical reaction can change or damage the DNA
strand. In most cases, a particular E. coli cell with mutated
DNA will either die, fix the damage in the strand or fail to
reproduce. In other words, most mutations go nowhere. But
every so often, a mutation will actually survive and the cell
Imagine, for example, a bunch of identical E. coli cells
that are living in a petri dish. With plenty of food and the
right temperature, they can double every 20 minutes. That is,
each E. coli cell can duplicate its DNA strand and split into
two new cells in 20 minutes.
Now, imagine that someone pours an antibiotic into the
petri dish. Many antibiotics kill bacteria by gumming up one
of the enzymes that the bacteria needs to live. For example,
one common antibiotic gums up the enzyme process that builds
the cell wall. Without the ability to add to the cell wall,
the bacteria cannot reproduce, and eventually they die.
When the antibiotic enters the dish, all of the bacteria
should die. But imagine that, among the many millions of
bacteria living in the dish, one of them acquires a mutation
that makes its cell-wall-building enzyme different from the
norm. Because of the difference, the antibiotic molecule does
not attach properly to the enzyme, and therefore does not
affect it. That one E. coli cell will survive, and since all
of its neighbors are dead, it can reproduce and take over the
petri dish. There is now a strain of E. coli that is immune to
that particular antibiotic.
In this example, you can see evolution at work. A random
DNA mutation created an E. coli cell that is unique. The cell
is unaffected by the antibiotic that kills all of its
neighbors. This unique cell, in the environment of that petri
dish, is able to survive.
E. coli are about as simple as living organisms can get,
and because they reproduce so rapidly you can actually see
evolution's effects on a normal time scale. In the past
several decades, many different types of bacteria have become
immune to antibiotics. In a similar way, insects
become immune to insecticides because they breed so quickly.
For example, DDT-resistant mosquitoes evolved from normal
In most cases, evolution is a much slower process.
The Speed of MutationsTherefore,
mutations are fairly common. Mutations happen at a steady rate
in any population, but the location and type of every mutation
is completely random. According to Carl Sagan in "The
Dragons of Eden":
As mentioned in the
previous section, many things can cause a DNA mutation,
organisms such as human beings average about one mutation
per ten gametes [a gamete is a sex cell, either sperm or
egg] -- that is, there is a 10 percent chance that any given
sperm or egg cell produced will have a new and inheritable
change in the genetic instructions that make up the next
generation. These mutations occur at random and are almost
uniformly harmful -- it is rare that a precision machine is
improved by a random change in the instructions for making
According to "Molecular
Biology of the Cell":
one nucleotide pair in a thousand is randomly changed every
200,000 years. Even so, in a population of 10,000
individuals, every possible nucleotide substitution will
have been "tried out" on about 50 occasions in the course of
a million years, which is a short span of time in relation
to the evolution of species. Much of the variation created
in this way will be disadvantageous to the organism and will
be selected against in the population. When a rare variant
sequence is advantageous, however, it will be rapidly
propagated by natural selection. Consequently, it can be
expected that in any given species the functions of most
genes will have been optimized by random point mutation and
According to the book "Evolution,"
by Ruth Moore, it is possible to speed up mutations with
So Muller put
hundreds of fruit flies in gelatin capsules and bombarded
them with X-rays. The irradiated flies were then bred to
untreated ones. In 10 days thousands of their offspring were
buzzing around their banana-mash feed, and Muller was
looking upon an unprecedented outburst of man-made
mutations. There were flies with bulging eyes, flat eyes,
purple, yellow and brown eyes. Some had curly bristles, some
Mutations fuel the process of evolution by providing new
genes in the gene
pool of a species.
Then, natural selection takes over.
As you saw in the previous
section, mutations are a random and constant process. As
mutations occur, natural selection decides which
mutations will live on and which ones will die out. If the
mutation is harmful, the mutated organism has a much decreased
chance of surviving and reproducing. If the mutation is
beneficial, the mutated organism survives to reproduce, and
the mutation gets passed on to its offspring. In this way,
natural selection guides the evolutionary process to
incorporate only the good mutations into the species, and
expunge the bad mutations.
The book "Extinct Humans," by Ian Tattersall and Jeffrey
Schwartz, puts it this way:
generation, many more individuals are produced than ever
survive to maturity and to reproduce themselves. Those that
succeed -- the "fittest" -- carry heritable features that
not only promote their own survival but are also passed
along preferentially to their offspring. In this view,
natural selection is no more than the sum of all those
factors that act to promote the reproductive success of some
individuals (and its lack in others). Add the dimension of
time, and over the generations natural selection will act to
change the complexion of each evolving lineage, as
advantageous variations become common in the population at
the expense of those less advantageous.
Let's look at an example of natural selection from How Whales
The ancestors of whales lived on land -- there is evidence
of the evolution of the whale from life on land to life in the
sea (read How
Whales Work for details), but how and why did this happen?
The "why" is commonly attributed to the abundance of food in
the sea. Basically, whales went where the food was. The "how"
is a bit more perplexing: Whales are mammals, like humans are,
and like humans, they lived and walked on solid ground,
breathing air into their lungs. How did whales become sea
creatures? One aspect of this evolution, according to Tom
Harris, author of How Whales
Work, is explained as follows:
To make this
transition, whales had to overcome a number of obstacles.
First of all, they had to contend with reduced access to
breathable air. This led to a number of remarkable
adaptations. The whale's "nose" moved from the face to the
top of the head. This blowhole makes it easy for whales to
breathe in air without fully surfacing. Instead, a whale
swims near the surface, arches its body so its back briefly
emerges and then flexes its tail, propelling it quickly to
Photo courtesy Sea World
Odd as it seems that the whale's "nose" actually changed
positions, the theory of evolution explains this phenomenon as
a long process that occurs over perhaps millions of years:
- Random mutation resulted in at least one whale
whose genetic information placed its "nose" farther back on
- The whales with this mutation were more suited to the
sea environment (where the food was) than "normal" whales,
so they thrived and reproduced, passing on this genetic
mutation to their offspring: Natural selection
"chose" this trait as favorable.
- In successive generations, further mutations placed the
nose farther back on the head because the whales with this
mutation were more likely to reproduce and pass on their
altered DNA. Eventually, the whale's nose reached the
position we see today.
Natural selection selects those genetic mutations that make
the organism most suited to its environment and therefore more
likely to survive and reproduce. In this way, animals of the
same species who end up in different environments can evolve
in completely different ways.
Creating a New Species
Imagine that you take
a group of Saint Bernards and put them on one island, and on
another island you put a group of Chihuahuas. Saint Bernards
and Chihuahuas are both members of the species "dog" right now
-- a Saint Bernard can mate with a Chihuahua (probably through
artificial insemination) and create normal puppies. They will
be odd-looking puppies, but normal puppies nonetheless.
Given enough time, it is possible to see how
speciation -- the development of a new species through
evolution -- could occur among the Saint Bernards and the
Chihuahuas on their respective islands. What would happen is
that the Saint Bernard gene pool would acquire random
mutations shared by all of the Saint Bernards on the island
(through interbreeding), and the Chihuahuas would acquire a
completely different set of random mutations shared by all of
the Chihuahuas on their island. These two gene pools would
eventually become incompatible with one another, to the point
where the two breeds could no longer interbreed. At that
point, you have two distinct species.
Because of the huge size difference between a Saint Bernard
and a Chihuahua, it would be possible to put both types of
dogs on the same island and have the exact same process occur.
The Saint Bernards would naturally breed with only the Saint
Bernards and the Chihuahuas would naturally breed with only
the Chihuahuas, so speciation would still occur.
If you put two groups of Chihuahuas on two separate
islands, the process would also occur. The two groups of
Chihuahuas would accumulate different collections of mutations
in their gene pools and eventually become different species
that could not interbreed.
The theory of evolution proposes that the process that
might create a separate Chihuahua-type species and Saint
Bernard-type species is the same process that has created all
of the species we see today. When a species gets split into
two (or more) distinct subsets, for example by a mountain
range, an ocean or a size difference, the subsets pick up
different mutations, create different gene pools and
eventually form distinct species.
Is this truly how all of the different species we see today
have formed? Most people agree that bacteria evolve in small
ways (microevolution), but there is some controversy
around the idea of speciation (macroevolution). Let's
take a look at where the controversy comes from.
Holes in the Theory
The theory of evolution
is just that -- a theory. According to "The American Heritage
Dictionary," a theory is:
A set of
statements or principles devised to explain a group of facts
or phenomena, especially one that has been repeatedly tested
or is widely accepted and can be used to make predictions
about natural phenomena.
is a set of principles that tries to explain how life, in all
its various forms, appeared on Earth. The theory of evolution
succeeds in explaining why we see bacteria and mosquitoes
becoming resistant to antibiotics and insecticides. It also
successfully predicted, for example, that X-ray exposure would
lead to thousands of mutations in fruit flies.
Many theories are works in progress, and evolution is one
of them. There are several big questions that the theory of
evolution cannot answer right now. This is not unusual.
Newtonian physics worked really well for hundreds of years,
and it still works well today for many types of problems.
However, it does not explain lots of things that were
eventually answered by Einstein and his theories of
relativity. People create new theories and modify existing
ones to explain the unexplained.
In answering the open questions that still remain unsolved,
the theory of evolution will either become complete or it will
be replaced by a new theory that better explains the phenomena
we see in nature. That is how the scientific process works.
Here are three common questions that are asked about the
current theory of evolution:
Let's look at each of these
questions briefly in the following sections.
- How does evolution add information to a genome to create
progressively more complicated organisms?
- How is evolution able to bring about drastic changes so
- How could the first living cell arise spontaneously to
get evolution started?
Question 1: How Does Evolution Add
The theory of evolution explains how
strands of DNA change. An X-ray, cosmic ray, chemical reaction
or similar mechanism can modify a base pair in the DNA strand
to create a mutation, and this modification can lead to the
creation of a new protein or enzyme.
The theory of evolution further proposes that billions of
these mutations created all of the life forms we see today. An
initial self-replicating molecule spontaneously formed. It
evolved into single-cell organisms. These evolved into
multi-cell organisms, which evolved into vertebrates like
fish, and so on. In the process, DNA structures evolved from
the asexual single-strand format found in bacteria today into
the dual-strand chromosomal format found in all higher life
forms. The number of chromosomes also proliferated. For
example, fruit flies have five chromosomes, mice have 20,
humans have 23 and dogs have 39.
Evolution's mutation mechanism does not explain how growth
of a genome is possible. How can point mutations create
new chromosomes or lengthen a strand of DNA? It is interesting
to note that, in all of the selective breeding in dogs, there
has been no change to the basic dog genome. All breeds of dog
can still mate with one another. People have not seen any
increase in dog's DNA, but have simply selected different
genes from the existing dog gene pool to create the different
One line of research in this area focuses on
transposons, or transposable elements, also referred to
as "jumping genes." A transposon is a gene that is able
to move or copy itself from one chromosome to another. The
Biology of the Cell" puts it this way:
elements have also contributed to genome diversity in
another way. When two transposable elements that are
recognized by the same site-specific recombination enzyme
(transposase) integrate into neighboring chromosomal sites,
the DNA between them can become subject to transposition by
the transposase. Because this provides a particularly
effective pathway for the duplication and movement of exons
(exon shuffling), these elements can help create new genes.
Another area of research involves polyploidy.
Through the process of polyploidy, the total number of
chromosomes can double, or a single chromosome can duplicate
itself. This process is fairly common in plants, and explains
why some plants can have as many as 100 chromosomes.
The amount of research in this area is truly remarkable and
is teaching scientists amazing things about DNA. The following
links give you a taste of that research, and are interesting
if you would like to learn more about these topics:
Question 2: How Can Evolution Be So
Imagine that you create a very large cage and
put a group of mice into it. You let the mice live and breed
in this cage freely, without disturbance. If you were to come
back after five years and look into this cage, you would find
mice. Five years of breeding would cause no change in the mice
in that cage -- they would not evolve in any noticeable way.
You could leave the cage alone for a hundred years and look in
again and what you would find in the cage is mice. After
several hundred years, you would look into the cage and find
not 15 new species, but mice.
The point is that evolution in general is an extremely slow
process. When two mice breed, the offspring is a mouse. When
that offspring breeds, its offspring is a mouse. When that
offspring breeds... And the process continues. Point mutations
do not change this fact in any significant way over the short
Carl Sagan, in "The Dragons of Eden," put it this way:
scale for evolutionary or genetic change is very long. A
characteristic period for the emergence of one advanced
species from another is perhaps a hundred thousand years;
and very often the difference in behavior between closely
related species -- say, lions and tigers -- does not seem
very great. An example of recent evolution of organ systems
in humans is our toes. The big toe plays an important
function in balance while walking; the other toes have much
less obvious utility. They are clearly evolved from
fingerlike appendages for grasping and swinging, like those
of arboreal apes and
monkeys. This evolution constitutes a
respecialization -- the adaptation of an organ system
originally evolved for one function to another and quite
different function -- which required about ten million years
The fact that it takes evolution 100,000 or 10 million
years to make relatively minor changes in existing structures
shows just how slow evolution really is. The creation of a new
species is time consuming.
On the other hand, we know that evolution can move
extremely quickly to create a new species. One example of the
speed of evolution involves the progress mammals have made.
You have probably heard that, about 65 million years ago, all
of the dinosaurs died out quite suddenly. One theory for this
massive extinction is an asteroid strike. For dinosaurs, the
day of the asteroid strike was a bad one, but for mammals it
was a good day. The disappearance of the dinosaurs cleared the
playing field of most predators. Mammals began to thrive and
Example: The Evolution of
65 million years ago, mammals were much
simpler than they are today. A representative mammal of the
time was the species Didelphodon,
a smallish, four-legged creature similar to today's opossum.
In 65 million years, according to the theory of evolution,
every mammal that we see today (over 4,000 species) evolved
from small, four-legged creatures like Didelphodon. Through
random mutations and natural selection, evolution has produced
mammals of striking diversity from that humble starting point:
Evolution has created thousands of
different species that range in size and shape from a small brown
bat that weighs a few grams to a blue whale that is nearly
100 feet (30.5 m) long.
- Panda bears
Let's take Carl Sagan's statement that "A characteristic
period for the emergence of one advanced species from another
is perhaps a hundred thousand years, and very often the
difference in behavior between closely related species -- say,
lions and tigers -- does not seem very great." In 65 million
years, there are only 650 periods of 100,000 years -- that's
650 "ticks" of the evolutionary clock.
Imagine trying to start with an opossum and get to an
elephant in 650 increments or less, even if every increment
were perfect. An elephant's brain is hundreds of times bigger
than an opossum's, containing hundreds of times more neurons,
all perfectly wired. An elephant's trunk is a perfectly formed
prehensile appendage containing 150,000 muscle elements (reference).
Starting with a snout like that of an opossum, evolution used
random mutations to design the elephant's snout in only 650
ticks. Imagine trying to get from an opossum to a brown bat in
650 increments. Or from an opossum to a whale. Whales have
no pelvis, have flukes, have very weird skulls (especially the
sperm whale), have blow holes up top, have temperature control
that allows them to swim in arctic waters and they consume
salt water rather than fresh. It is difficult for many people
to imagine that sort of speed given the current theory.
Example: The Evolution of the Human
Here is another example of the speed problem.
evidence indicates that modern humans evolved from a species
called Homo erectus. Homo erectus appeared about 2 million
years ago. Looking at the skull of Homo erectus, we know that
its brain size was on the order of 800 or 900 cubic
Modern human brain
size averages about 1,500 CCs or so. In other words, in about
2 million years, evolution roughly doubled the size of the
Homo erectus brain to create the human brain that we have
today. Our brains contain approximately 100 billion
neurons today, so in 2 million years, evolution added 50
billion neurons to the Homo erectus brain (while at the same
time redesigning the skull to accommodate all of those neurons
and redesigning the female pelvis to let the larger skull
through during birth, etc.).
Let's assume that Homo erectus was able to reproduce every
10 years. That means that, in 2 million years, there were
200,000 generations of Homo erectus possible. There are four
possible explanations for where the 50 billion new neurons
came from in 200,000 generations:
- Every generation, 250,000 new neurons were added to the
Homo erectus brain (250,000 * 200,000 = 50 billion).
- Every 100,000 years, 2.5 billion new neurons were added
to the Homo erectus brain (2,500,000,000 * 20 = 50 billion).
- Perhaps 500,000 years ago, there was a spurt of 20 or so
closely-spaced generations that added 2.5 billion neurons
- One day, spontaneously, 50 billion new neurons were
added to the Homo erectus brain to create the Homo sapiens
None of these
scenarios is particularly comfortable. We see no evidence that
evolution is randomly adding 250,000 neurons to each child
born today, so that explanation is hard to swallow. The
thought of adding a large package of something like 2.5
billion neurons in one step is difficult to imagine, because
there is no way to explain how the neurons would wire
themselves in. What sort of point mutation would occur in a
DNA molecule that would suddenly create billions of new
neurons and wire them correctly?* The current theory of
evolution does not predict how this could happen.
|* In an
absolutely facinating experiment first reported in July
2002, scientists modified a single mouse gene and
created mice with brains 50% larger than normal. See this
article for details. This experiment shows that a
point mutation can, in fact, have an immense effect on
brain size. It is still unknown whether the larger
brains make the mice smarter or not, but it is easy to
imagine later mutations refining the wiring of these
millions of new neurons.
fascinating study, researches have identified
minimal changes in an amino acid on a single gene that
have a profound effect on speech processing in humans.
It does appear that tiny changes in single genes can
have very large effects on the species.
One line of current research is looking at the effect of
very small changes in DNA patterns during embryonic
development. Any new animal, be it a mouse or a human, starts
life as a single cell. That cell differentiates and develops
into the complete animal. A tremendous amount of signaling
happens between cells during the development process to ensure
that everything ends up in the right place. Tiny changes in
these signaling processes can have very large effects on the
resulting animal. This is how the human genome, with at most
60,000 or so genes, is able to specify the creation of a human
body containing trillions of cells, billions of carefully
wired neurons and hundreds of different cell types all
brilliantly sculpted into organs as diverse as the heart and
The book "Molecular Biology of the Cell" puts it this way:
Humans, as a
genus distinct from the great apes, have existed for only a
few million years. Each human gene has therefore had the
chance to accumulate relatively few nucleotide changes since
our inception, and most of these have been eliminated by
natural selection. A comparison of humans and monkeys, for
example, shows that their cytochrome-c molecules differ in
about 1 percent and their hemoglobins in about 4 percent of
their amino acid positions. Clearly, a great deal of our
genetic heritage must have been formed long before Homo
sapiens appeared, during the evolution of mammals (which
started about 300 million years ago) and even earlier.
Because the proteins of mammals as different as whales and
humans are very similar, the evolutionary changes that have
produced such striking morphological differences must
involve relatively few changes in molecules from which we
are made. Instead, it is thought that the morphological
differences arise from differences in the temporal and
spatial pattern of gene expression during embryonic
development, which then determine the size, shape and other
characteristics of the adult.
In other words, there just are not that many differences in
the DNA of a human and a whale, yet humans and whales look
totally different. Small collections of DNA mutations can have
a very big effect on the final result.
Right now, the signaling mechanisms that wire up the 100
billion cells in the human brain are something of a mystery.
How can the mere 60,000 genes in the human genome tell 100
billion neurons how to precisely wire themselves in the human
brain? No one right now has a clear understanding of how so
few genes can meticulously wire so many neurons. In a
developing fetus in the womb, DNA is correctly creating and
wiring up millions of cells per minute. Given that DNA
does wire up a working human brain every time a baby is
born, it may be the case that DNA has special properties that
make evolution work more efficiently. As the mechanisms become
better understood, the effects of DNA mutations during
development will become better understood as well.
Question 3: Where Did the First Living Cell Come
In order for the principles of mutation and
natural selection in the theory of evolution to work, there
have to be living things for them to work on. Life must exist
before it can to start diversifying. Life had to come from
somewhere, and the theory of evolution proposes that it arose
spontaneously out of the inert chemicals of planet Earth
perhaps 4 billion years ago.
Could life arise spontaneously? If you read How Cells
Work, you can see that even a primitive cell like an E.
coli bacteria -- one of the simplest life forms in existence
today -- is amazingly complex. Following the E. coli model, a
cell would have to contain at an absolute minimum:
- A cell wall of some sort to contain the cell
- A genetic blueprint for the cell (in the form of DNA)
- An enzyme capable of copying information out of the
genetic blueprint to manufacture new proteins and enzymes
- An enzyme capable of manufacturing new enzymes, along
with all of the building blocks for those enzymes
- An enzyme that can build cell walls
- An enzyme able to copy the genetic material in
preparation for cell splitting (reproduction)
- An enzyme or enzymes able to take care of all of the
other operations of splitting one cell into two to implement
reproduction (For example, something has to get the second
copy of the genetic material separated from the first, and
then the cell wall has to split and seal over in the two new
- Enzymes able to manufacture energy molecules to power
all of the previously mentioned enzymes
Obviously, the E. coli cell itself is the product of
billions of years of evolution, so it is complex and intricate
-- much more complex than the first living cells. Even so, the
first living cells had to possess:
Otherwise, it is not really a cell and it is not
really alive. To try to imagine a primordial cell with these
capabilities spontaneously creating itself, it is helpful to
consider some simplifying assumptions. For example:
- A cell wall
- The ability to maintain and expand the cell wall (grow)
- The ability to process "food" (other molecules floating
outside the cell) to create energy
- The ability to split itself to reproduce
These examples do simplify the
requirements for the "original cell," but it is still a long
way to spontaneous generation of life. Perhaps the first
living cells were completely different from what we see today,
and no one has yet imagined what they might have been like.
Coming up with an explanation for where the first cell came
from is important to the theory of evolution, because life can
only have come from one of two possible places:
- Perhaps the original energy molecule was very different
from the mechanism found in living cells today, and the
energy molecules happened to be abundant and free-floating
in the environment. Therefore, the original cell would not
have had to manufacture them.
- Perhaps the chemical composition of the Earth was
conducive to the spontaneous production of protein chains,
so the oceans were filled with unimaginable numbers of
random chains and enzymes.
- Perhaps the first cell walls were naturally forming
lipid spheres, and these spheres randomly entrapped
different combinations of chemicals.
- Perhaps the first genetic blueprint was something other
- Spontaneous creation - Random chemical processes created
the first living cell.
- Supernatural creation - God or some other supernatural
power created the first living cell.
And it doesn't really matter if aliens or meteorites
brought the first living cell to earth, because the aliens
would have come into existence through either spontaneous
creation or supernatural creation at some point -- something
had to create the first alien cells.
Most likely, it will be many years before research can
completely answer any of the three questions mentioned here.
Given that DNA was not discovered until the 1950s, the
research on this complicated molecule is still in its infancy,
and we have much to learn.
The Future of Evolution
One exciting thing
about the theory of evolution is that we can see its effects
both today and in the past. For example, the book "Evolution"
known reptiles are so amphibian-like that their assignment
to one category or the other is largely a matter of opinion.
In this area of life, however, there was no missing link;
all the gradations from amphibian to reptile exist with a
clarity seldom equaled in paleontology.
In other words, there is plenty of evidence, past and
present, for some sort of evolutionary process. We see it in
bacteria and insects today, and we see it in the fossil record
through the development of millions of species over millions
After thinking about questions like the three mentioned in
the previous sections, different people come to different
conclusions. In the future, there are three possible scenarios
for the theory of evolution:
- Scientists will come to a complete understanding of DNA
and show how mutations and natural selection explain every
part of the development of life on this planet.
- Scientists will develop a new theory that answers the
questions posed above to almost everyone's satisfaction, and
it will replace the theory of evolution that we have today.
- Scientists will observe a completely new phenomenon that
accounts for the diversity of life that we see today. For
example, many people believe in creationism. In this
theory, God or some other supernatural power intervenes to
create all of the life that we see around us. The fossil
record indicates that hundreds of millions of new species
have been created over hundreds of millions of years --
Species creation is an intense and constant process with an
extremely long history. If scientists were to observe the
creation process occurring the next time a major new species
comes into existence, they could document it and understand
how it works.
Let's assume that the theory of evolution as currently
stated is the process that did bring about all of the life
that we see today. One compelling question is: "What happens
next?" Evolution must be at work right now. Our species, Homo
sapiens, only appeared about 40,000 years ago. What does
evolution have in store for human beings, and how will the
change manifest itself?
These are all fascinating questions to
think about. They reveal just how big an effect evolution can
have. Given enough time, evolution could completely alter life
on this planet by disposing of the species we see today and
creating new ones.
- Will a child appear one day whose brain is twice as big
as any normal human brain? If so, what will be the
capabilities of that brain, and how will it differ from the
brain seen today? Or are our brains slowly evolving right
- Will children appear one day who have more than 23
chromosomes? If so, what will be the effects of the new
- Will man learn how to control or accelerate evolution
through genetic engineering? Once we completely understand
different genomes, will we be able to engineer evolutionary
steps that lead to new species on a much faster schedule?
What would those species look like? What would we design
them to do?
For lots more information on evolution and related topics,
check out the links on the next page.
Lots More Information!
More Great Links
Sites About Evolution
Sites Against Evolution
Transposons and Polyploidy
Abiogenesis and RNA World
Humans," by Ian Tattersall, Jeffrey Schwartz
The Spark of Life: Darwin and the Primeval Soup," by
Christopher Wills, Jeffrey Bada
Dragons of Eden: Speculations on the Evolution of Human
Intelligence," by Carl Sagan
Darwinism by Opening Minds," by Phillip E. Johnson
by Ruth Moore & Time-Life Books
Biology of the Cell," by Bruce Alberts, et al.
with Dinosaurs: A Natural History," by Tim Haines
of Man: The Story of Human Evolution," by Robin McKie
Your Own Vegetable Varieties," by Carol Deppe
Naked Ape: A Zoologist's Study of the Human Animal," by
Blind Watchmaker," by Richard Dawkins
Structure of Evolutionary Theory," by Stephen Jay Gould
Home in the Universe: The Search for Laws of
Self-Organization and Complexity," by Stuart Kauffman
Black Box: The Biochemical Challenge to Evolution," by
Michael J. Behe
Origin of Species," by Charles Darwin
Ghost: The Origin of Species Updated," by Steve Jones
Four Billion Years of Evolution from Our Microbial
Ancestors," by Lynn Margulis, et al
of Evolution: Science or Myth?," by Jonathan Wells, Jody
F. Sjogren (Illustrator)