Thousands of scientists, mostly paleontologists, working independently have identified fossils of varying complexity and of widely differing ages. When organized by common features, they form a heirarchy of life referred to as the "tree of life".
This brief "Tree of Life" shows how all of life (flora, fauna, bacteria) evolved from a single cell organism..
From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so they can be classed in groups under groups.Charles Darwin, Origin of Species, Chapter 13
FOSSIL TREE OF LIFE
The first tree of life was developed by study of the fossil record over years of fossil discovery. Many scientistis, over 150 years have participated in the accumulation of the knowledge that has culminated in the FOSSIL RECORD tree of life.
To see a very detailed, interactive, well presented tree of life as worked out by geologists, palentologists, go to this web site.
Here is another website that again, does a great job of presenting this material, with pictures (if you drill down) and approximate dates,