Cells and Living Organisms
Almost all life on Earth is made of cells. Cells are the most basic unit of life.
Unicellular Organisms
Unicellular organisms consist of just one cell. This organism uses this single cell to carry out all of life functions. A unicellular organism grows as the cell increases in size.
Multicellular Organisms
Multicellular organisms have many cells that are specialized to and have specialized functions to support the organism. Multicellular organisms grow as the number of their cells increase.
Now, as we look further into the question “What is Life,” there are many hypotheses as to how life came to be. Many scientists feel as though the beginnings of life started with unicellular organisms. In the infamous words of Jurassic Park consultant Dr. Ian Malcolm, “Life breaks free. Life expands to new territories. Painfully perhaps even dangerously, but life finds a way”. 4 The topic of life beginning on planets is ripe for discussion because scientists have found out that bacteria existed on Earth before any mammals resided here let alone humans. The Earth did not see any indication of life before the Holocene Period which is when signs of life were first detected. Life on Earth began about 3.5 to 4 billion years ago and the 1st life forms were single celled organisms similar to bacteria according to the Geologic Time Scale (See table 1 Below).
Table 1: The Timeline of 4.6 billion years of Planet Earth:
|
Period |
Major Events |
|
Hadean |
Unicellular life appears |
|
Archean |
Photosynthesis begins |
|
Proterzoic |
1st eukaryotes, 1st multicellular life |
|
Cambrian |
The Cambrian Explosion |
|
Ordovician |
Molluscs and arthropods |
|
Silurian |
Land Plants |
|
Devonian |
Bony fish |
|
Carboniferous |
Coal Age swamps, amphibians, and insects |
|
Permian |
Reptiles |
|
Triassic |
Dinosaurs arrive |
|
Jurassic |
Dinosaurs dominate |
|
Cretaceous |
Dinosaur extinction |
|
Paleocene |
Early mammals |
|
Exocene |
Warm, wet climate, modern mammal families |
|
Oligocene |
Drying period, continents nearing current position |
|
Miocene |
Hominids appear |
|
Pliocene |
Cooling Period |
|
Pleistocene |
Major Ice Age |
|
Holocene |
Modern humans arrive |
https://ucmp.berkeley.edu/education/explorations/tours/geotime/gtpage2c.html
Comments: