More recently, studies of DNA and other more elusive information have revealed some surprises. Living species are now categorized into a group ABOVE Kingdom, the Domain. Like so:

And the old Five Kingdom System you probably knew in high school also has been replaced!

And all of these new classifications are based on studies of which species evolved most recently from a common ancestor. In other words--who's most closely related to whom! You can read a bit more about this HERE.

The earliest organisms (bacteria and archaebacteria) were very simple PROKARYOTIC cells, containing little but genetic material (DNA or RNA), cytoplasm, and protein-making machinery called ribosomes:

The direct descendants of these cells still exist today as Domain Bacteria and Domain Archaebacteria, all of which are unicellular and prokaryotic organisms we commonly call "bacteria" (though this name is more descriptive than accurately reflecting evolutionary relationships).

Archaebacteria are believed to be the first organisms to have evolved on earth, and they share a more recent common ancestor with eukaryotes than the true bacteria (Domain Bacteria) do.

Later, eukaryotic cells evolved from prokaryotic cells via two hypothetical processes, the Autogenous Model (shown in "a" below) and the Endosymbiont Model (shown in "b" below)...

...and evolutionary processes (we'll discuss later) caused the divergence of these cells into the diversity of life we see around us today.

Domain Eukarya includes all the eukaryotic organisms:

• Protists (several Kingdoms) - unicellular and colonial eukaryotes
• Kingdom Fungi - The Fungi (multicellular eukaryotes; heterotrophic decomposers)
• Kingdom Plantae - The Plants (multicellular eukaryotes; autotrophs)
• Kingdom Animalia - The Animals (multicellular eukaryotes; ingestive heterotrophs)

What is an ANIMAL?

• It's a multicellular eukaryote
• It has tissues made up of specialized cells
• Those tissues make up organs, each with a specific function
• The organs are associated into ORGAN SYSTEMS that serve a common purpose
  • integumentary
  • digestive
  • respiratory
  • circulatory
  • muscular (unique to animals)
  • nervous (unique to animals)
  • endocrine
  • reproductive
  (though not all animals have all these systems)
  
• It's an ingestive heterotroph (i.e., it eats other living things to obtain energy)
• It stores energy long term as fat; short term as glycogen ("animal starch")
• It reproduces sexually (though a few can reproduce asexually).
• It has a characteristic EMBRYONIC DEVELOPMENT

And the diversity of Kingdom Animalia is amazing!