Unit 4. The origin and evolution of life

Science for the Contemporary World

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.

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.

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

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.

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.

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

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

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.

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.

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

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.

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.

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.

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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.

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.

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.

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

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.

Anatomic proofs. Compared anatomy

Analogous organs:

≠ structure

= function

CONVERGENT EVOLUTION

Homologous organs:

= structure

≠ function

DIVERGENT EVOLUTION