An
organism
is an individual
living
thing. It is easy to recognize a living thing, but not so easy to define it.
Animals
and
plants
are organisms, obviously. Organisms are a
biotic
, or living, part of the
environment
. Rocks and sunshine are parts of the non-living
environment
.
Organisms usually have six basic needs to continue their
metabolism
. They need
air
,
water
,
nutrient
(
food
),
energy
, a place to live, and
homeostatsis
(being able to maintain itself). However, not all living things need all these at the same time. Some organisms do not need access to air at all.
The characteristics of living things are if they have cells, take and use energy, grow and develop, share similar chemicals, sense and resond to change (stimulus), and if they reproduce.
A little thought is needed about
viruses
. There is no agreement as to whether they should be regarded as living. They are made of
protein
and
nucleic acid
, and they
evolve
, which is a really important fact. However, they exist in two quite different phases. One phase is
dormant
, not active. The other is inside a living
cell
of some other organism. Then the virus is very active reproducing itself. Consider the parallel with a
computer program
. When in use it is active; when it is not, it is completely inactive. It is still a program all the same.
Another example from biology is the
spore
, which is a distribution phase of a
bacteria
,
fungus
or some plants. They are not active until they get to the right situation. They have all the working parts to build a complete organism, but for the moment it is switched off.
Some organisms are made up of millions of
cells
. They are
multicellular organisms
. Many can be seen without using a microscope.
Most organisms are so small that they cannot be seen with the naked eye. You need a
microscope
to see them. They are called
microorganisms
. Organisms can be made up of just one cell. They are called
unicellular organisms
or
single celled organisms
. Examples include
bacteria
, and
protozoa
such as the
Amoeba
and
Paramecium
.
The
Tree of Life
project works on the relationships between living things. Identifying a
LUCA
(last universal common ancestor) is one of its main aims. The LUCA is estimated to have lived some 3.8
billion
years ago (sometime in the
Palaeoarchaean
era
).
[1]
[2]
- A universal common ancestor is at least 10
2860
times more probable than having multiple ancestors.
[3]
- A model with a single common ancestor but allowing for some gene swapping among species was... 10
3489
times more probable than the best multi-ancestor model...
[3]
The idea came from
Charles Darwin
's
On the Origin of Species
, "Therefore... probably all the organic beings which have ever lived on this earth have descended from some one primordial form..."
- ↑
Doolittle, W.F. (2000),
"Uprooting the tree of life"
(PDF)
,
Scientific American
,
282
(6): 90?95,
Bibcode
:
2000SciAm.282b..90D
,
doi
:
10.1038/scientificamerican0200-90
,
PMID
10710791
, archived from
the original
(PDF)
on 2011-01-31
, retrieved
2013-06-06
.
- ↑
Glansdorff, N.; Xu, Y; Labedan, B. (2008), "The last universal common ancestor: emergence, constitution and genetic legacy of an elusive forerunner",
Biology Direct
,
3
: 29,
doi
:
10.1186/1745-6150-3-29
,
PMC
2478661
,
PMID
18613974
.
- ↑
3.0
3.1
Hesman Saey, T. (2010).
"All modern life on Earth derived from common ancestor"
. Discovery News. Archived from
the original
on 2012-08-25
. Retrieved
2013-06-06
.