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The Earliest Fossils

The Earth is teaming with life. Most of the planet's inhabitants are far removed from their genetic ancestors. In fact, as much as 99 percent of all known life has become extinct in the more than billion years that have passed since Earth's primordial beginnings. While it is impossible to make direct observations of the planet's deep past, fossil records do provide an idea of the way things were millions and billions of years in the past.

Earliest Known Life Forms
- Bacteria are among the oldest known forms of life.
  
  Colonies of cyanobacteria, which have also been known as blue-green algae, deposited their fossilized remains as much as 3.465 billion years ago. These tiny remains, known as microfossils, have been found in coral reels from the Apex Chert, Australia. Similar microfossils, dating back to the same early period in the history of life on Earth, have also been discovered in South Africa, Canada and other parts of Australia. The estimated age of these fossils makes these lifeforms the oldest known.
Earliest Dinosaurs
- Dinosaurs great and small roamed the Earth until 65 million years ago.
  
  For many, the word "fossil" conjures up images of massive dinosaur skeletons towering several stories high; but next to cyanobacteria the dinosaur is a comparatively new form of life. Fossils dated between 235 and 240 million years old, found by Jorge Ferigolo in Grande de Sul, have shed some light on the earliest known period in dinosaur history.
  
  It was during this time that some palaeontologists believe primitive four-legged reptiles began to specialize and adopt bipedal forms which lent them greater speed and agility. These new forms continued to evolve over tens of millions more years, becoming the various massive reptiles known today as dinosaurs; they thrived all across the planet until extinction overtook them 65 million years ago.
Earliest Human Ancestors
- The earliest known humans became distinct from neanderthals 195,000 years ago.
  
  Exactly when humans became what they are today is a point of controversy. In 1967, a team led by Richard Leaky identified the ancient remains of modern humans near Ethiopia's Omo River. At the time, they dated the fossils as 130,000 years old, but they were unsure over the accuracy of their dating technique. A newer study, which was published in the journal Nature in February, 2005, utilized more modern dating techniques to establish the fossils' age at 195,000 years old, making them the oldest known evidence of humanity's early existence.
Living Fossils
- Horshoe crabs have evolved little, if at all, over millions of years.
  
  Some sharks, insects, crustaceans, Coelacanth fish and bacteria are known as so-called living fossils because of their identical or near-identical appearance to fossils from millions of years ago. While it would be difficult to demonstrate whether or not these modern throwbacks to ancient times are genetically identical to their fossilized ancestors, their striking physical similarities make a good case to support the idea that some life forms reached an optimal stage in their evolutionary advancement and, being perfectly adapted to survive in their respective environments, reached a point of evolutionary stagnation because no further evolutionary advantage could be gained by further specialization.

Animals & Science