It was so simple: "animal," "vegetable,"
"mineral." Plants produced food, animals ate plants—and
each other, minerals provided the building blocks.
Of course, there were some awkward fits. The non-photosynthetic
fungi clearly weren't animals, but also were not clearly plants. And what
about the one-celled organisms that moved and made their own food?
Our natural inclination to name and group things helps us to
communicate with one another about those things and serves as an
interpretation of the relationships we perceive among those things.
How we organize the world we observe is determined in great part by which
details we can observe and the importance we place on those details—and
on the roles the organisms play in our lives. "Plant" and
"animal" started to get dicey when our microscopes could discern
the minute details of one-celled living things that moved and had packets
of chlorophyll.
Currently, it is broadly held that life on our planet is best divided
into Five Kingdoms: Monera, Protoctista, Fungi, Plants, Animals.
A key element in the current life classification system is whether or
not the organisms' cells are compartmentalized, forming a nucleus (and
sometimes other organelles) set off by its own membrane. Organisms
without a nucleus are "prokaryotes;" organisms with a nucleus
are "eukaryotes." Other differentiating characteristics
are whether or not the organisms can make their own food, and whether or
not the organisms can move on their own.
Monera are small and relatively simple, with one-cell that does
not have a nucleus or other organelles. Some make their own food,
others do not; some are mobile, others are not. Examples include
bacteria and blue-green bacteria. (Some researchers advocate
splitting this Kingdom into two.)
Protoctista (sometimes called Protista) are larger and more
complex than Monera—each cell has its own nucleus. Most are
single-celled, a few are multi-cellular. Some make their own food,
others do not; some are mobile, others are not. Examples include
protozoans, such as amoeba and paramecium, and one-celled algae, such as
diatoms, as well as multi-cellular algae, such as kelp. (The
Protoctista Kingdom is sometimes viewed as a catch-all group for organisms
that don't fit well in the other four Kingdoms.)
Fungi are organisms with many
cells, each with a separate nucleus, that cannot produce their own food
and cannot move. Examples include mushrooms, mildew, and yeast.
Plants are organisms with many cells, each with a separate
nucleus, that can (usually) produce their own food and (usually) cannot
move. Examples include moss, cedars, and strawberries.
Animals are organisms with many cells, each with a separate
nucleus, that cannot produce their own food and can (usually) move.
Examples include earthworms, crabs, and people.
Each Kingdom is further divided into phyla or divisions—the name
varies with the Kingdom. There are seven layers of taxonomic
divisions in total, each layer identifying a level of grouping:
Kingdom, Phylum/Division, Class, Order, Family, Genus, Species.
Now, what about viruses? And the
recently identified prions (such as those apparently responsible for Mad
Cow Disease)? Those groups of replicating materials are not cells
nor do they perform any metabolic functions on their own, so they are not
included in the current Five Kingdoms of living organisms.
The relationships between the groups change, too. Fungi, once
placed in the Plant Kingdom, are now thought by many to be more closely
related to the animals.
As we make better observations about living things, we improve our
understanding and adapt our way of grouping them.
It seems that our inexhaustible quest for knowledge about our world
gathers bits that make the picture more complex before it becomes more
clear.
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Visit our pages on related topics:
scientific names
messy nature
viruses
fungi
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Wavecrest Discoveries can craft your personal discovery of this
delightful part of our world by customizing one of our distinctive guided
excursions. Our walks, tours,
and special activities are wonderful ways to
explore this fascinating region—and are the
perfect entertainment for guests.
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