Hadean Eon

The Precambrian fossil record is poor, and what fossils are present are of little use for biostratigraphic work. Many Precambrian rocks are heavily metamorphosed, obscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.

Answer Expert Verified Am certain that the factor that prevents us from finding fossils from the Precambian era is that Most rocks from that era are deeply buried. The precambrian is the informal name for the large expanse of time preceding the current Phanerozoic Eon.

Due to the fact that fossils from the Precambrian Period are limited, very few fossils can be found in Precambrian-aged rocks and these mostly of single celled organisms that don’t change much over long periods of times.

Why do scientists not divide Precambrian time into smaller units based on the fossil record? Geologic changes have made it hard to interpret the Precambrian fossil record, so scientists cannot divide Precambrian time into smaller units. Precambrian organisms did not have hard parts such as bones or shells.

4,000 million years ago

2,500 million years ago

During the Proterozoic, the earth had cooled considerably from the previous Hadean eon when the planet was covered by molten lava. Near the end of the Proterozoic, ice sheets were growing towards the equator, and the entire planet was possibly engulfed under a thin layer of snow and ice.

Although the first continents formed during this eon, rock of this age makes up only 7% of the present world’s cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5–40% of the present area of continents formed during the Archean.

The Proterozoic Eon is the most recent division of the Precambrian. It is also the longest geologic eon, beginning 2.5 billion years ago and ending 541 million years ago.

The Archean Eon (4 to 2.5 billion years ago)

about 700 million years

The Hadean Eon, named after the Greek god and ruler of the underworld Hades, is the oldest eon and dates from 4.5–4.0 billion years ago. This time represents Earth’s earliest history, during which the planet was characterized by a partially molten surface, volcanism, and asteroid impacts.

Fossils are rare in Precambrian rocks, probably because Precambrian life-forms lacked bones, shells, or other hard parts that commonly form fossils. Also, Precambrian rocks are extremely old.

The age of these rocks was estimated using a radiometric dating technique that measures the ratio of the rare-earth elements neodymium and samarium present in a sample.

Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events.

sedimentary rock

Gaining estimates of ages of rocks is crucial for establishing not only the history of geological events but also for determining the rates of geological processes. It is possible to establish the relative order of events in some rocks.

Preserved remains become fossils if they reach an age of about 10,000 years. Fossils can come from the Archaeaean Eon (which began almost 4 billion years ago) all the way up to the Holocene Epoch (which continues today).

Mostly, however, heavy and lightly colored objects are rocks, like flint. Paleontologists also examine the surfaces of potential fossils. If they are smooth and do not have any real texture, they are probably rocks. Even if it is shaped like a bone, if it does not have the right texture then it is probably a rock.

Fossils are found in sedimentary rocks and hold the clues to life on Earth long ago. Limestone: You humans would know so little without rocks like me. After all, fossils of extinct animals like dinosaurs and woolly mammoths are found in sedimentary rocks.

Scientists discovered what they thought were 3.5 billion-year-old fossils in western Australia almost 40 years ago. A new study reveals that these rocks did indeed contain organic life — making them the oldest fossils ever found.

Smallpox and measles viruses are among the oldest that infect humans. Having evolved from viruses that infected other animals, they first appeared in humans in Europe and North Africa thousands of years ago.

Mimivirus is the largest and most complex virus known.

Viruses may have arisen from mobile genetic elements that gained the ability to move between cells. They may be descendants of previously free-living organisms that adapted a parasitic replication strategy. Perhaps viruses existed before, and led to the evolution of, cellular life.

Mycobacterium tuberculosis (MTB) may well be the oldest pathogen to haveinfected humankind. Modern humans (or homo sapiens) emerged out of the “hominid” group almost two million years ago, and began wandering out of Africa about 70,000 years ago to populate the world.