Origin of Life: The Heterotroph Hypothesis
Life on Earth began about 3.5 billion years ago. At that
point in the development of the Earth, the atmosphere was very different
from what it is today. As opposed to the current atmosphere, which
is mostly nitrogen and oxygen, the early Earth atmosphere contained mostly
hydrogen, water, ammonia, and methane.
In experiments, scientists have showed that the electrical
discharges of lightning, radioactivity, and ultraviolet light caused
the elements in the early Earth atmosphere to form the basic molecules
of biological chemistry, such as nucleotides, simple proteins, and
ATP. It seems likely, then, that the Earth was covered in a hot,
thin soup of water and organic materials. Over time, the molecules
became more complex and began to collaborate to run metabolic processes.
Eventually, the first cells came into being. These cells
were heterotrophs, which could not produce their own
food and instead fed on the organic material from the primordial
soup. (These heterotrophs give this theory its name.)
The anaerobic metabolic processes of the heterotrophs
released carbon dioxide into the atmosphere, which allowed for the
evolution of photosynthetic autotrophs, which could use
light and CO2 to produce
their own food. The autotrophs released oxygen into the atmosphere.
For most of the original anaerobic heterotrophs, oxygen proved poisonous. The
few heterotrophs that survived the change in environment generally
evolved the capacity to carry out aerobic respiration. Over the
subsequent billions of years, the aerobic autotrophs and heterotrophs
became the dominant life-forms on the planet and evolved into all
of the diversity of life now visible on Earth.
Evidence of Evolution
Humankind has always wondered about its origins and the
origins of the life around it. Many cultures have ancient creation
myths that explain the origin of the Earth and its life. In Western
cultures, ideas about evolution were originally based on the Bible.
The book of Genesis relates how God created all life on Earth about
6,000 years ago in a mass creation event. Proponents of creationism
support the Genesis account and state that species were created
exactly as they are currently found in nature. This oldest formal
conception of the origin of life still has proponents today.
However, about 200 years ago, scientific evidence began
to cast doubt on creationism. This evidence comes in a variety of
Rock and Fossil Formation
Fossils provide the only direct evidence of the history
of evolution. Fossil formation occurs when sediment covers some
material or fills an impression. Very gradually, heat and pressure
harden the sediment and surrounding minerals replace it, creating
fossils. Fossils of prehistoric life can be bones, shells, or teeth
that are buried in rock, and they can also be traces of leaves or
footprints left behind by organisms.
Together, fossils can be used to construct a fossil
record that offers a timeline of fossils reaching back through
history. To puzzle together the fossil record, scientists have to
be able to date the fossils to a certain time period. The strata
of rock in which fossils are found give clues about their relative
ages. If two fossils are found in the same geographic location, but
one is found in a layer of sediment that is beneath the other layer,
it is likely that the fossil in the lower layer is from an earlier
era. After all, the first layer of sediment had to already be on
the ground in order for the second layer to begin to build up on
top of it. In addition to sediment layers, new techniques such as
radioactive decay or carbon dating can also help determine a fossil’s
There are, however, limitations to the information fossils
can supply. First of all, fossilization is an improbable event.
Most often, remains and other traces of organisms are crushed or
consumed before they can be fossilized. Additionally, fossils can
only form in areas with sedimentary rock, such as ocean floors.
Organisms that live in these environments are therefore more likely
to become fossils. Finally, erosion of exposed surfaces or geological
movements such as earthquakes can destroy already formed fossils.
All of these conditions lead to large and numerous gaps in the fossil
Scientists often try to determine the relatedness of two
organisms by comparing external and internal structures. The study
of comparative anatomy is an extension of the logical reasoning
that organisms with similar structures must have acquired these
traits from a common ancestor. For example, the flipper of a whale
and a human arm seem to be quite different when looked at on the
outside. But the bone structure of each is surprisingly similar,
suggesting that whales and humans have a common ancestor way back
in prehistory. Anatomical features in different species that point
to a common ancestor are called homologous structures.
However, comparative anatomists cannot just assume that
every similar structure points to a common evolutionary origin.
A hasty and reckless comparative anatomist might assume that bats
and insects share a common ancestor, since both have wings. But
a closer look at the structure of the wings shows that there is
very little in common between them besides their function. In fact,
the bat wing is much closer in structure to the arm of
a man and the fin of a whale than it is to the wings of an insect.
In other words, bats and insects evolved their ability to fly along
two very separate evolutionary paths. These sorts of structures,
which have superficial similarities because of similarity of function
but do not result from a common ancestor, are called analogous
In addition to homologous and analogous structures, vestigial
structures, which serve no apparent modern function, can
help determine how an organism may have evolved over time. In humans
the appendix is useless, but in cows and other mammalian
herbivores a similar structure is used to digest cellulose. The
existence of the appendix suggests that humans share a common evolutionary
ancestry with other mammalian herbivores. The fact that the appendix
now serves no purpose in humans demonstrates that humans and mammalian
herbivores long ago diverged in their evolutionary paths.
Homologous structures not present in adult organisms often do appear
in some form during embryonic development. Species that bear little
resemblance to each other in their adult forms may have strikingly
similar embryonic stages. In some ways, it is almost as if the embryo
passes through many evolutionary stages to produce the mature organism.
For example, for a large portion of its development, the human embryo
possesses a tail, much like those of our close primate relatives.
This tail is usually reabsorbed before birth, but occasionally children
are born with the ancestral structure intact. Even though they are
not generally present in the adult organism, tails could be considered
homologous traits between humans and primates.
In general, the more closely related two species are,
the more their embryological processes of development resemble each
Just as comparative anatomy is used to determine the anatomical
relatedness of species, molecular biology can be used to determine
evolutionary relationships at the molecular level. Two species that
are closely related will have fewer genetic or protein differences between
them than two species that are distantly related and split in evolutionary
development long in the past.
Certain genes or proteins in organisms change
at a constant rate over time. These genes and proteins, called molecular
clocks because they are so constant in their rate of change,
are especially useful in comparing the molecular evolution of different
species. Scientists can use the rate of change in the gene or protein
to calculate the point at which two species last shared a common
ancestor. For example, ribosomal RNA has a very slow rate of change,
so it is commonly used as a molecular clock to determine relationships
between extremely ancient species. Cytochrome c, a protein that
plays an important role in aerobic respiration, is an example of
a protein commonly used as a molecular clock.
Theories of Evolution
In the nineteenth century, as increasing evidence suggested
that species changed over time, scientists began to develop theories
to explain how these changes arise. During this time, there
were two notable theories of evolution. The first, proposed by Lamarck,
turned out to be incorrect. The second, developed by Darwin, is
the basis of all evolutionary theory.
Lamarck: Use and Disuse
The first notable theory of evolution was proposed by
Jean-Baptiste Lamarck (1744–1829). He described a two-part mechanism
by which evolutionary change was gradually introduced into the species
and passed down through generations. His theory is referred to as
the theory of transformation or Lamarckism.
The classic example used to explain Lamarckism is the
elongated neck of the giraffe. According to Lamarck’s theory, a
given giraffe could, over a lifetime of straining to reach high
branches, develop an elongated neck. This vividly illustrates Lamarck’s
belief that use could amplify or enhance a trait.
Similarly, he believed that disuse would cause
a trait to become reduced. According to Lamarck’s theory, the wings
of penguins, for example, were understandably smaller than the wings
of other birds because penguins did not use their wings to fly.
The second part of Lamarck’s mechanism for evolution involved
the inheritance of acquired traits. He believed that
if an organism’s traits changed over the course of its lifetime,
the organism would pass these traits along to its offspring.
Lamarck’s theory has been proven wrong in both of its
basic premises. First, an organism cannot fundamentally change its
structure through use or disuse. A giraffe’s neck will not become
longer or shorter by stretching for leaves. Second, modern genetics
shows that it is impossible to pass on acquired traits; the traits
that an organism can pass on are determined by the genotype of its
sex cells, which does not change according to changes in phenotype.
Darwin: Natural Selection
While sailing aboard the HMS Beagle,
the Englishman Charles Darwin had the opportunity to study the wildlife
of the Galápagos Islands. On the islands, he was amazed by the great
diversity of life. Most particularly, he took interest in the islands’
various finches, whose beaks were all highly adapted to their particular
lifestyles. He hypothesized that there must be some process that
created such diversity and adaptation, and he spent much of his
time trying to puzzle out just what the process might be. In 1859,
he published his theory of natural selection and the evolution it
produced. Darwin explained his theory through four basic points:
- Each species produces more offspring than
- The individual organisms that make up a larger population
are born with certain variations.
- The overabundance of offspring creates a competition for
survival among individual organisms. The individuals that have the
most favorable variations will survive and reproduce, while those
with less favorable variations are less likely to survive and reproduce.
- Variations are passed down from parent to offspring.
Natural selection creates change within a species
through competition, or the struggle for life. Members of a species
compete with each other and with other species for resources. In
this competition, the individuals that are the most fit—the
individuals that have certain variations that make them better adapted
to their environments—are the most able to survive, reproduce, and
pass their traits on to their offspring. The competition that Darwin’s
theory describes is sometimes called the survival of the fittest.
Natural Selection in Action
One of the best examples of natural selection is a true
story that took place in England around the turn of the century.
Near an agricultural town lived a species of moth. The moth spent
much of its time perched on the lichen-covered bark of trees of
the area. Most of the moths were of a pepper color, though a few
were black. When the pepper-color moths were attached to the lichen-covered
bark of the trees in the region, it was quite difficult for predators
to see them. The black moths were easy to spot against the black-and-white
The nearby city, however, slowly became industrialized.
Smokestacks and foundries in the town puffed out soot and smoke
into the air. In a fairly short time, the soot settled on everything,
including the trees, and killed much of the lichen. As a result,
the appearance of the trees became nearly black in color. Suddenly
the pepper-color moths were obvious against the dark tree trunks,
while the black moths that had been easy to spot now blended in
against the trees. Over the course of years, residents of the town
noticed that the population of the moths changed. Whereas about
90 percent of the moths used to be light, after the trees became
black, the moth population became increasingly black.
When the trees were lighter in color, natural
selection favored the pepper-color moths because those moths were
more difficult for predators to spot. As a result, the pepper-color moths
lived to reproduce and had pepper-color offspring, while far fewer
of the black moths lived to produce black offspring. When the industry
in the town killed off the lichen and covered the trees in soot,
however, the selection pressure switched. Suddenly the black moths
were more likely to survive and have offspring. In each generation,
more black moths survived and had offspring, while fewer lighter
moths survived to have offspring. Over time, the population as a
whole evolved from mostly white in color to mostly black in color.
Types of Natural Selection
In a normal population without selection pressure, individual
traits, such as height, vary in the population. Most individuals
are of an average height, while fewer are extremely short or extremely
tall. The distribution of height falls into a bell curve.
Natural selection can operate on this population in three
basic ways. Stabilizing selection eliminates extreme
individuals. A plant that is too short may not be able to compete
with other plants for sunlight. However, extremely tall plants may
be more susceptible to wind damage. Combined, these two selection
pressures act to favor plants of medium height.
Directional selection selects against one
extreme. In the familiar example of giraffe necks, there was a selection
pressure against short necks, since individuals with short necks
could not reach as many leaves on which to feed. As a result, the
distribution of neck length shifted to favor individuals with long
Disruptive selection eliminates
intermediate individuals. For example, imagine a plant of extremely
variable height that is pollinated by three different pollinator
insects: one that was attracted to short plants, another that preferred
plants of medium height, and a third that visited only the tallest
plants. If the pollinator that preferred plants of medium height
disappeared from an area, medium height plants would be selected
against, and the population would tend toward both short and tall
plants, but not plants of medium height.
The Genetic Basis for Evolution
Darwin’s theory of natural selection and evolution rests
on two crucial ideas:
Variations exist in the individuals within a population.
variations are passed down from one generation to the next.
But Darwin had no idea how those variations came
to be or how they were passed down from one generation to the next.
Mendel’s experiments and the development of the science of genetics
provided answers. Genetics explains that the phenotype—the physical
attributes of an organism—is produced by an organism’s genotype.
Through the mechanism of mutations, genetics explains how variations
arose among individuals in the form of different alleles of genes.
Meiosis, sexual reproduction, and the inheritance of alleles explain
how the variations between organisms are passed down from parent
With the modern understanding of genes and inheritance,
it is possible to redefine natural selection and evolution in genetic
terms. The particular alleles that an organism inherits from its
parents determine that organism’s physical attributes and therefore
its fitness for survival. When the forces of natural selection result
in the survival of the fittest, what those forces are really doing
is selecting which alleles will be passed on from one generation
to the next.
Once you see that natural selection is actually a selection
of the passage of alleles from generation to generation, you can
further see that the forces of natural selection can change the
frequency of each particular allele within a population’s gene
pool, which is the sum total of all the alleles within a
particular population. Using genetics, one can create a new definition
of evolution as the change in the allele frequencies in
the gene pool of a population over time. For example, in the population
of moths we discussed earlier, after the trees darkened, the frequency
of the alleles for black coloration increased in the gene pool,
while the frequency of alleles for light coloration decreased.
The Hardy-Weinberg principle states that a sexually reproducing
population will have stable allelic frequencies and therefore will
not undergo evolution, given the following five conditions:
- large population size
- no immigration or emigration
- random mating
- random reproductive success
- no mutation
The Hardy-Weinberg principle proves that variability and
inheritance alone are not enough to cause evolution; natural selection
must drive evolution. A population that meets all of these conditions
is said to be in Hardy-Weinberg equilibrium. Few natural
populations ever experience Hardy-Weinberg equilibrium, though,
since large populations are rarely found in isolation, all populations
experience some level of mutation, and natural selection simply
cannot be avoided.
Development of New Species
The scientific definition of a species is
a discrete group of organisms that can only breed within its own
confines. In other words, the members of one species cannot interbreed with
the members of another species. Each species is said to experience reproductive
isolation. If you think about evolution in terms of genetics,
this definition of species makes a great deal of sense: if species
could interbreed, they could share gene flow, and their evolution
would not be separate. But since species cannot interbreed, each
species exists on its own individual path.
As populations change, new species evolve. This process
is known as speciation. Through speciation, the earliest
simple organisms were able to branch out and populate the world
with millions of different species. Speciation is also called divergent
evolution, since when a new species develops, it diverges
from a previous form. All homologous traits are produced by divergent
evolution. Whales and humans share a distant common ancestor. Through
speciation, that ancestor underwent divergent evolution and gave
rise to new species, which in turn gave rise to new species, which
over the course of millions of years resulted in whales and humans.
The original ancestor had a limb structure that, over millions of
years and successive occurrences of divergent evolution, evolved
into the fin of the whale and the arm of the human.
Speciation occurs when two populations become reproductively
isolated. Once reproductive isolation occurs for a new species,
it will begin to evolve independently. There are two main ways in
which speciation might occur. Allopatric speciation occurs
when populations of a species become geographically isolated so
that they cannot interbreed. Over time, the populations
may become genetically different in response to the unique selection
pressures operating in their different environments. Eventually
the genetic differences between the two populations will become
so extreme that the two populations would be unable to interbreed even
if the geographic barrier disappeared.
A second, more common form of speciation is adaptive
radiation, which is the creation of several new species from
a single parent species. Think of a population of a given species, which
we’ll imaginatively name population 1. The population moves into
a new habitat and establishes itself in a niche, or role, in the
habitat (we discuss niches in more detail in the chapter on Ecology).
In so doing, it adapts to its new environment and becomes different
from the parent species. If a new population of the parent species,
population 2, moves into the area, it too will try to occupy the
same niche as population 1. Competition between population 1 and
population 2 ensues, placing pressure on both groups to adapt to
separate niches, further distinguishing them from each other and
the parent species. As this happens many times in a given habitat,
several new species may be formed from a single parent species in
a relatively short time. The immense diversity of finches that Darwin observed
on the Galápagos Islands is an excellent example of the products
of adaptive radiation.
When different species inhabit similar environments, they
face similar selection pressures, or use parts of their bodies to
perform similar functions. These similarities can cause the species
to evolve similar traits, in a process called convergent evolution.
From living in the cold, watery, arctic regions, where most of the
food exists underwater, penguins and killer whales have evolved
some similar characteristics: both are streamlined to help them
swim more quickly underwater, both have layers of fat to keep them
warm, both have similar white-and-black coloration that helps them
to avoid detection, and both have developed fins (or flippers) to
propel them through the water. All of these similar traits are examples of
analogous traits, which are the product of convergent evolution.
Convergent evolution sounds as if it is the opposite of
divergent evolution, but that isn’t actually true. Convergent evolution
is only superficial. From the outside, the fin of a whale may look
like the flipper of a penguin, but the bone structure of a whale
fin is still more similar to the limbs of other mammals than it
is to the structure of penguin flippers. More importantly, convergent
evolution never results in two species gaining the ability to interbreed;
convergent evolution can’t take two species and turn them into one.