There have been a number of schemes for classifying the relationships between organisms.

Originally three kingdoms, plants, animals and single-celled monera were proposed.

Later, a five kingdom scheme of animals, plants, fungi, protozoa, and bacteria was popular, and can be still found in some textbooks.

In the light of a growing body of molecular data, however, a three kingdom model has emerged. These three kingdoms are the Archaea, the Bacteria and the Eukarya.

Plants, animals, fungi and protozoa are all members of the Eukarya.

For example, some organisms can to pick up genes from other organisms, a process known as horizontal gene transfer or HGT.

HGT is responsible for the rapid spread of antibiotic resistance in modern bacteria and has probably played an key roll in the early evolution of life.

At the root of the tree of life is the common unit of life, the cell.

The basic and univeral features of cells were well established in the last common ancestor.

Cells are bounded by a plasma membrane that controls the movement of molecules into and out of the cytoplasm.

The cytoplasm contains the molecular machinery needed for life. It supports the processes involved in capturing energy and building a new cell

Within the cytoplasm is the genetic material DNA, which stores the information needed to build a new cell and to respond to environmental changes.

Bacteria and archaea were originally lumped together as "prokaryotes", meaning before or without a nucleus.

Their DNA lies within the cytoplasm in a structure known as a nucleoid.

From a gross structural view bacteria and archaea can be difficult to distinguish.

Most of the organisms we are aware of, the macroscopic plants and animals we see around us, are eukarya.

The defining feature of the eukarya is a distinct, double membrane-bounded organelle, the nucleus within the cytoplasm of the cell.

A nuclear envelope separates the nuclear interior from the cytoplasm.

The nuclear envelope is fenestrated by nuclear pores that mediate the movement of molecules into and out of the nucleus.

The cell's genetic material lies within the nucleus.

The eukarya are hybrid organisms. They contain cytoplasmic organelles, known as mitochondria, which are present in all eukarya, and chloroplasts, which are characteristic of plants and other photosynthetic eukarya.

Both mitochondria and chloroplasts are descended from what were once free-living organisms, bacteria in fact.

This conclusion is based on the observation that mitochondria and chloroplasts contain chromosomes and ribosomes that are similar to those found in bacteria, and distinct from those found in archaea or eukarya.

Whether the event that lead to the formation of the eukarya involved the 'simple' fusion of two distinct organisms or was the end result of a more long-term endosymbiotic relationship remains unclear.

Mitochondrial network (green) inside a eukaryotic cell; the nucleus is the large dark circular structure

From the ~3000 to 4000 genes in a typical bacterium, a mitochondrion now contain less than 100 genes and can no longer survive outside of its"partner".

• Legionella pneumophila
• Listeria monocytogenes
• Toxoplasma gondii
• Mycobacterium tuberculosis
• Chlamydia psittaci
• Salmonella typhi
• Plasmodium falciparum

Bacteria and archaea arose between 3.5 to 3.0 billion years ago.

It took another 1.5 billion years for the eukarya to appear.

The period before the appearance of multicellular organisms is known as the Proterozoic Era.

Larger multicellular organisms did not appear until ~650 million years ago, although microscopic multicellular organisms may have been present much earlier.

The earliest multicellular organisms appeared during the Vendian period, which lasted from 650 to 544 million years ago. These are known as Vendian or Ediacarian organisms.

Exactly how they are related to modern multicellular organisms remains a subject of debate.

It is also unclear what triggered the appearance of multicellular organisms after so many years.

The earth has gone through many dramatic changes during its history.

One hypothesis, the "snowball" earth hypothesis, proposes that ~600 million years ago the entire surface of the earth was covered with ice.

The earth was 'rescued' from this snowball state by an drastic increase in "greenhouse" gases that lead to a global warming up.

Drastic climate changes are assumed to have produced conditions that favored the appearance of multicellular organisms.

In any case, by the Cambrian age, which began about 544 million years ago, metazoans or multicellular animals and metaphyta or multicellular plants had appeared in abundance.

If you think about it, there are only a few paths that lead from unicellular to multicellular organisms.

The colonial route involves cells that either fail to separate after division or that clumped together.

Alternatively, an single-celled organism containing multiple nuclei, a type of cell known as a syncytium, could divide to form distinct cells.

Shown here is a artist's drawing of the only known placozoan, Trichoplax adhaerens. Its surface is covered with motile appendages, known as cilia.

Whether these organisms are primitive or degenerate, however, remains to be determined.

It is worth remembering that while evolution implies change over time, it does necessarily mean increasingly complexity.

There are many situations in which simplification can lead to a reproductive advantage.

It is possible that placozoans are degenerate, that is they are derived from more complex organisms and have evolved to reach their current, apparently primitive state.

All modern metazoans appear to share a common ancestor.

This conclusion, originally based on structural similarities, has been repeatedly confirmed by molecular analyses.

All metazoans share common 'signaling' and genetic pathways by which they control embryonic development.

Given these molecular similarities it likely that all of these signaling systems were present in the common ancestor of the metazoans.