Evolution: from Unicellular to Multicellular Organisms

The first living organisms on earth 3.5 billion years ago were microscopic unicellular (single-celled) organisms. Larger multicellular organisms did not evolve from these microscopic organisms until 3 billion years later.

Many of the same biological and chemical processes that developed in the early microscopic unicellular organisms are still seen in today's microbes and higher order organisms.

Unicellular Organisms

Most single-celled (unicellular) organisms are microscopic - too small to be seen with the 'naked' eye. These microorganisms were the first living organisms to develop on the earth approximately 3.5 billion years ago. And for the next 3 billion years, the earth was only inhabited by microscopic organisms.

Specialized Cells in Multicellular Organisms

Most of the living organisms that can be seen with the unaided eye are multicellular. This includes all animals (i.e. members of the kingdom Animalia) and plants (i.e. members of the kingdom Plantae).

Unlike single-celled organisms, the cells in multicellular organisms perform specialized physiological functions. Humans, for example, have more than 250 types of cells that have different functions - from nerve cells (neurons) to liver cells. Cells that make up the human skin are different from cells that make up the inner organs.

Stem and Multipotent Cells - Cells That Can Differentiate into Specialized Cells

Stem cells (in animals) and meristematic cells (in plants) are unspecialized cells that can develop (differentiate) into many different cell types during early life and growth. These cells can divide continuously, replenishing other cells as long as the organism is alive.

When a stem cell divides, new cells can either remain as stem cells or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.

Embryonic cells can differentiate into any kind of cell. However adult stem cells can develop in only a few - if any - types of cells.

Pluripotent cells have the potential to develop into most types of specialized cells. Embryonic cells are pluripotent.

Totipotent cells are cells that can regenerate whole new individuals eg cloning. In mammals, only the zygote and early embryonic cells are totipotent, while in plants, many cells can become totipotent with simple laboratory procedures and techniques.

Multipotent or progenitor cells can differentiate but only into a closely related family of cells. For example, a blood stem cell can develop into different kinds of blood cells but not any kind of brain cell.

Comparison between Stem and Meristematic Cell Types

Pluripotent	Multipotent	totipotent
Plants	Animals	Plants	Animals	Plants	Animals
Meristematic cells	Stem cells	Meristematic cells; Differentiated cells have potential	Blood stem cells	Differentiated cells have potential	Zygotes Early embryonic cells

Pluripotent

Multipotent

totipotent

Plants

Animals

Plants

Animals

Plants

Animals

Meristematic cells

Stem cells

Meristematic cells; Differentiated cells have potential

Blood stem cells

Differentiated cells have potential

Zygotes

Early embryonic cells