Environmental Engineering Reference
In-Depth Information
There is a grandeur to this view of life ...that, whilst
this planet has gone cycling on ...endless forms most
beautiful and most wonderful have been, and are being,
evolved.
C
HARLES
D
ARWIN
4-1
ORIGINS OF LIFE
Development of Life
on the Primitive Earth
Scientific evidence indicates that the earth's life is the
result of about 1 billion years of chemical evolution to
form the first cells, followed by about 3.7 billion years
of biological evolution to form the species we find on
the earth today.
How did life on the earth evolve to its present incredi-
ble diversity of species? We do not know the full an-
swers to these questions. But considerable evidence
suggests that life on the earth developed in two phases
over the past 4.6-4.7 billion years.
The first phase involved
chemical evolution
of the
organic molecules, biopolymers, and systems of chem-
ical reactions needed to form the first cells. It took
about 1 billion years. Evidence for this phase comes
from chemical analysis and measurements of radioac-
tive elements in primitive rocks and fossils. Chemists
have also conducted laboratory experiments showing
how simple inorganic compounds in the earth's early
atmosphere might have reacted to produce amino
acids, simple sugars, and other organic molecules used
as building blocks for the proteins, complex carbohy-
drates, RNA, and DNA needed for life.
This chapter describes how scientists believe life on
earth arose and developed into the diversity of species
we find today. It discusses these questions:
How do scientists account for the development of
life on the earth?
■
What is biological evolution by natural selection,
and how has it led to the current diversity of
organisms on the earth?
■
What is an ecological niche, and how does it help a
population adapt to changing environmental
conditions?
■
How do extinction of species and formation of new
species affect biodiversity?
■
What is the future of evolution, and what role
should we play in this future?
■
KEY IDEAS
■
Chemical, geological, and fossil evidence indicate
that life on earth developed as a result of about 1 bil-
lion years of chemical evolution and 3.7 billion years
of biological evolution.
Learn more about one of the most famous of these experi-
ments exploring early life at Environmental ScienceNow.
Fossil and other evidence indicates that chemical
evolution was followed by
biological evolution
from
single-celled bacteria to multicellular protists, plants,
fungi, and animals.
■
Populations can evolve when genes change or
mutate, giving some individuals genetic traits that en-
hance their ability to survive and to produce offspring
with these traits (natural selection).
How Do We Know Which Organisms Lived
in the Past?
Our knowledge about past life comes from fossils,
chemical analysis, cores drilled out of buried ice, and
DNA analysis.
Most of what we know of the earth's life history comes
from
fossils:
mineralized or petrified replicas of skele-
tons, bones, teeth, shells, leaves, and seeds, or impres-
sions of such items. Fossils provide physical evidence
of ancient organisms and reveal what their internal
structures looked like (Figure 4-2).
Despite its importance, the fossil record is uneven
and incomplete. Some forms of life left no fossils,
some fossils have decomposed, and others are yet to
be found. The fossils we have found so far are be-
lieved to represent only 1% of all species that have
ever lived.
■
Each species in an ecosystem fills a unique ecologi-
cal role called its ecological niche.
■
As enviroinmental conditions change, the balance
between the formation of new species (speciation)
and the disapearance or extinction of existing ones
determines the earth's biodiversity.
■
Human activities decrease the earth's biodiversity
when they cause the premature extinction of species
and destroy or degrade habitats that traditionally
served as centers for the development of new
species.
■
Genetic engineering enables us to combine genes
from different organisms—a process that both holds
great promise and raises a number of legal, ethical,
and environmental issues.
