All matter is composed of atoms, and atoms in turn tend to bond together to form molecules. These molecules thereby acquire new properties that the atoms alone did not possess. Such properties are called emergent properties.
For example, some molecules, such as lipids, have parts that
are repelled by water, or “hydrophobic”. These
molecules tend to turn their hydrophobic parts to one another,
to avoid contact with water. A double layer of lipids thus
forms. This layer can close on itself to create an internal
environment and an external environment... in other words,
a cell membrane.The cell can then begin to develop
more complex structures.
HOW LIFE ON EARTH BEGAN
According to the theory of the Big Bang,
the universe as we know it was born about 14 billion years ago (12
to 16 according to the methods of calculation). Our sun and the planets
formed much later: about 4.6 billion years ago. The earliest traces
of living things on Earth date back 3.8 billion years. Thus, we can
say that life on Earth began some time between 4.6 and 3.8 billion
The appearance of life
on Earth was part of a broader process that had been going
on since the Big Bang: an increase in the complexity of matter.
At some point in time, the structure of the molecules of which
matter was composed became so complex that they took on new
properties, those of living things. At this point, chemical
evolution left off and biological evolution began.
Source: University of California,
Center for Astrophysics and Space Sciences
The first living things must have resembled the bacteria present
on Earth today. These early organisms had three new basic characteristics:
- a permeable membrane that separated them from
the outside world;
- genetic material that enabled them to replicate
- proteins that constituted the actual building
blocks of life, for which the assembly instructions were
contained in these organisms’ genetic material.
All living cells, whether
they come from a mouse or a whale, a bacterium or a banyan
tree, have three things in common. All of them have a cell
membrane, proteins, and a genetic code made up of DNA.
The universality of this genetic code is one
of the strongest pieces of evidence for evolution and
for the common relationship among all living things.
Genes are long sequences of DNA that contain the instructions
for making proteins. Some of these genes are present
in identical form both in human beings and in far less
evolved life forms, such as shellfish.
The molecules that
compose living organisms are made up of the same atoms
as the molecules that compose non-living matter. Thus organic
molecules differ not in the nature of their components,
but rather in the way that these components are arranged.
Organic molecules do, however, show a marked tendency to
contain certain atoms, such as carbon, oxygen, hydrogen and
nitrogen. Organic molecules also contain smaller but still
appreciable amounts of calcium, phosphorus, potassium, sulphur,
sodium and magnesium.
With the possible exception of the simplest atom, hydrogen,
which formed during the first few moments that the universe
existed, most of the atoms found in the human body were
formed through fusion of simpler atoms in the intensely hot
centres of early stars. When these stars died, they exploded,
ejecting vast clouds of all these atoms out into space. In
this way, the evolution of the cosmos provided the elements
without which life itself could never have evolved.
First of all, water accounts for about 80% of the mass of every form of life.
This is because the first living cells originated in the water.
Second, the complex molecules that characterize
living organisms are made up of simpler molecules that constitute
the basic building blocks of life.
For example, lipids, or fatty acids, are
the molecules from which cell membranes are made. Simple
sugars, the cells preferred energy source, are stored
in long chains of complex sugars.
which form the structure of cells and promote
certain essential chemical reactions, are
composed of chains of 20 different smaller
molecules called amino acids.
These chains can be several hundred amino
acids long. The chains forming any given
protein are twisted together in a unique
way that gives that protein its function.
The genetic material in each cell contains the coded instructions
for all the components of that cell and is the only part
of the cell that can replicate itself. The basic building
blocks for this genetic material are called nucleotides.
Just four nucleotides form the long chains of DNA
and RNA. It is the sequence of these nucleotides on these
chains that provides the genetic information needed to build
all the other components of the cell.