1. Unit 4. The origin and
evolution of life
Science for the Contemporary World
2. 1. The bases of life
What is life? Which qualities distinguish an organism from inert matter?
These are very complex questions that have had many different interpretations in the course
of the history, from a religious, phylosophical or scientifical point of view.
Then, it is much more useful to wonder which are the characteristics of living organisms and
what’s the point to consider that an organism is alive.
1.1 Properties of organisms
Organisms share several characteristics:
- They are complex
- They are made of cells (one single cell or many cells)
- They perform the 3 vital functions: relation, nutrition and reproduction.
3. Organisms are also made of atoms. Those chemical elements in the periodic table that are
part of living organisms are called biolements.
Bioelements can be distinguished depending on their proportion in living organisms:
- Primary bioelements: indispensable to form organic biomolecules. They
represent the 96% of elements in living organisms: carbon (C), hydrogen (H), oxygen (O),
nitrogen (N), phosphorus (P) and sulphur (S).
- Secondary bioelements : less abundant but as important as the others: iron (Fe),
calcium (Ca), potassium (K), sodium (Na), etc.
4. Biomolecules are those molecules belonging to living organisms. They can be classified
regarding their chemical composition (whether they have carbonic backbones or not):
a) Inorganic biomolecules :
- Water
- Mineral salts
b) Organic biomolecules: their backbone is made of C atoms.
- Glucides
- Lipids
- Proteins
- Nucleic acids
5. 2. The origins of life
Spontaneous generation: The first explanation of the origin of life are of mythological and
supernatural: The gods are the creators of life. However, with the arrival of Greek civilization
begins to prevail and philosophy appears Aristotelian theory of spontaneous generation.
According to it, life arises spontaneously by the combination of four basic elements of nature:
water, earth, air and fire.
But Redi disprove the theory of spontaneous generation through a simple experiment.Redi
took six jars, which he divided in two groups of three: In one experiment, in the first jar of each
group, he put an unknown object; in the second, a dead fish; in the last, a raw chunk of veal.
Redi took the first group of three, and covered the tops with fine gauze so that only air could
get into it. He left the other group of jars open. After several days, he saw maggots appear on
the objects in the open jars, on which flies had been able to land, but not in the gauze-
covered jars. In the second experiment, meat was kept in three jars. One of the jars was
uncovered, and two of the jars were covered, one with cork and the other one with gauze.
Flies could only enter the jar with the lid off of it, and in this, maggots appeared. In the jar that
were covered with gauze, maggots stayed on the gauze and did not live.
Cell theory: “The cell is the functional unit of life, and all living things are made of cells that
come from other preexisting cells". This theory demonstrated the inaccuracy of spontaneous
generation, like Pasteur experiments carried out in 1860. Pasteur introduced a culture broth in
a flask, container long neck. He heated the neck of the flask to bending in a gooseneck,
leaving its open end. The contents of the flask boiled to kill any life form. It allowed to cool the
broth. Air entered, but no microorganisms were deposited in the broth.
6. Primordial soup: Oparin argued that the Earth's early atmosphere had to undergo a chemical
evolution prior to the emergence of life. The primitive atmospheric gases (methane,
hydrogen, ammonia and water) reacted with each other when cooling occurred ground,
causing a large amount of organic molecules (monomers) carbon, hydrogen, oxygen and
nitrogen. They were joined in the hydrosphere, constituting the primordial soup. Miller
confirmed experimentally
Oparin's hypothesis: In a round bottom flask presumably introduced gases were primitive
atmosphere (methane, hydrogen, ammonia and water vapor) and subjected to electric
shocks storms simulating possible for one week. Vessel kept at a temperature close to the
boiling point of water and found that simple organic molecules appeared as carbohydrates,
fatty acids and amino acids (protein precursors).
Other hypotheses: In addition to the hypothesis of the early atmosphere there are 3 other less
accepted by the scientific community:
- Metabolic hypothesis: simple molecules were isolated by a membrane and began a series
of metabolic chemical reactions, leading to the living.
- WORLD RNA: RNA molecules emerged simple random and began an evolution by
successive mutations. In the first stage occurs RNA self-replication and subsequent protein
binding generates DNA. From DNA and RNA binding proteins arise new chain needed to
continue.
- PANSPERMIA: The first forms of life came from space. Today, it´s still being studied.
7. 3. The diversity of organisms: the evolution
We define biodiversity as the variety of living organisms inhabiting the planet at a specific
time point. Nowadays, it is believed that more than 50-60 million species live in our planet,
while only 1,2 million have been already discovered.
Once scientist realized about the biodiversity of our planet, next questions were:
How can be explained such a huge biodiversity?
Why organisms are different between them?
But the most important question to the scientific community was…
… where do current organisms come from?
During this subject, we’ll study different theories that try to answer this:
- Fixist theories
- Evolutionist theories
8. 3.1 Fixist theories
Different fixist theories state the same:
- Living organisms appeared independently
- Living organisms did no change along the time. They remain completely the
same as in the past and as they will be in the future.
- Living organisms have been created by a divine creator.
In general, fixist theories describe nature as a finished and definitive reality, and they deny the
possibility of a common origin for all living organisms.
There are two different theories in fixism:
- Creationism
- Catastrophism
9. CREATIONISM
-It deny species extinction: organisms are not actually extinguished, but they live in unknown
habitats. Thus, it can not explain the existence of fossils.
10. CATASTROPHISM
Naturalists in XVIII found a huge amount of fossils (the 99% of species that have inhabited
the Earth are extinguished). The paleontology was born.
Since the creationism could not explain the existence of fossils, in 1798, naturalist Georges
Cuvier (the father of paleontology) proposed a modification of the creationism: the
catastrophism:
Fossils are a group of organisms that were extinguished in big catastrophes. The last was
The Flood. Surviving species have kept invariable until today.
11. 3.2 Evolutionist theories
They defend that current species come from previous species, thanks to the accumulation of
gradual and successive changes along time.
There are several evolutionist theories:
- Lamarckism
- Darwinism
- Neodarwinism
Jean-Baptiste Lamarck Charles Darwin
12. A previous idea; the concept of speciation
- We define specie as a group of individuals that resemble among them and that can
reproduce obtaining fertile descendants.
… but, how can a new specie raise from a preexistent one?
The speciation process:
1. A population of individuals from the same specie share the habitat.
2. A geographic, ecologic or ethologic barrier appears, separating two
subgroups that do not breed anymore. Each of the groups gets adapted to the
new environment (adaptive modifications).
3. After several generations, the differences are so noticeable that individuals
from both subgroups can not reproduce anymore = new specie.
13. THE LAMARCKISM: THE ADAPTATION TO THE ENVIRONMEN
Lamarck stated that species can change along the time, as they get better adapted to the
environment. In nature, there constantly occur changes in the physical medium, that cause
new necessities in living organisms. To survive, they have to be adapted to these changes,
through “internal changes” that generate the necessary changes in the organism
(hypertrophy, atrophy, appearance of new organs, etc).
The gradual accumulation of these changes along the time causes the rising of new species.
The two main statements of Lamarckism are:
USE AND DISUSE: Individuals lose characteristics they do not require (or use) and develop
characteristics that are useful.This gain or lose of characteristics is due to the effort of
organisms.
Ex: giraffes that made an effort to reach leaves of the top of the trees. Their neck become
longer and longer.
INHERITANCE OF ACQUIRED TRAITS: Individuals inherit the traits acquired thanks to the
effort of their ancestors.
Ex: the long neck was transmitted to the offspring through several generations. At the end, all
giraffes had long necks.
14.
15. DARWINISME: THE NATURAL SELECTION
Lamarck did not provide any proof nor performed any experiment to demonstrate his
statements. Theory proposed by the british naturalist Charles Darwin in its famous “On the
Origin of Species” (1859). There he also proposed that current species come from preexistent
organisms, but in an opposite way from Lamarck’s theory.
Even some modifications, darwinism still constitutes one of the pillars of modern Biology, and
his discoveries are a logical explanation that unifies all the observations about life diversity.
.
16. Darwin aboard the HMS Beagle (1831-1836)
Darwin took part in the scientific expedition in the HMS Beagle (1831-36), travelling around
the world for five years. Darwin collected a huge number of samples and data about living
organisms from diverse regions, being the most crucial the study of fauna in Galapagos
Islands: similar species with very specific differences, caused by the adaptation to different
environments.
17. Darwin:
- observed variations in the beak of different finch species
- observed variations in the shape and colour of the shell in different turtle
species
DARWINISM: THE NATURAL SELECTION
Through the analysis of the observations and data obtained along its trip, Darwin
proposed its theory, mainly based in two principal statements:
a) VARIABILITY IN THE OFFSPRING
b) NATURAL SELECTION: Natural selection is a pressure that selects those
organisms well adapted and tends to eliminate the others.
In a given population, individuals are not identical to their parents neither between them.
Instead, they present a small variability. Darwin did not known the origin of this
variety, so he attribute it to chance. Under unfriendly conditions (competence for
food, light, etc), only those who are the best adapted will survive. They will be able
to breed, transmitting these beneficial characters to their offspring. Generation after
generation, more and more individuals of the specie will show these characters,
very suitable to survive. Then, individuals carrying beneficial variations will survive
better than the others.
18. NEODARWINISM: THE ADDITION OF GENETIC ADVANCES
a) VARIABILITY IN THE OFFSPRING
The results obtained by Gregor Mendel and, later, by Hugo de Vries, created the concept of
mutation: inheritable changes in the genetic material. They are caused by chance, although
several factors such as radiations and chemical substances can trigger them. Ex: tobacco
smoke, UV radiations, etc.
It is known that there are positive, negative and neutral mutations. Natural selection will tend
to favor the positives (they improve the adaptation and survival of the individual) and eliminate
the negatives (they are harmful for the organism).
b) TRANSMISSION OF GENETIC INFORMATION
Darwin observed that those characteristics improving the adaptation and survival of the
specie were transmitted from parents to their offspring. However, he did not understand how
this mechanism worked.
In 1953, James Watson and Francis Creek discovered the structure of de DNA molecule.
That was crucial to understand how the DNA contains the genetic information and how it is
transmitted to descendants.
19. 3.3 The evolution proofs
As we already studied, the Evolution Theory proposes that current species appeared from
preexistent species. Its confirmation is based on a variety of empirical proofs coming from
several scientific disciplines:
- Paleontological proofs
- Anatomical proofs
- Embryological proofs
- Biochemical proofs
20. 1. Paleontological proofs.
They are proofs obtained by the analysis of the fossil register. They demonstrate
that:
-There is an increase in the complexity along time: most ancient fossils
correspond to the simplest organisms.
-There is an increase of diversity along time: there is few variety of very ancient
fossilized organisms, whereas there is a high number of more recent fossil
species.
-There are fossils with intermediate characteristics, that allow us to
understand the evolution from one specie to the other.
-There are living fossils: current organisms with ancient appearances, that look
like how would be their ancestors. They help us to understand the morphological
and physiological characteristics of ancient species.
