1.during which period did the atmosphere reach its current oxygen rich state? 2.during which period did land organisms appear how many millions of years ago did this take place? during which period did the first primates appear . during which period did the humans appear?

1.Cambrian period (541 million years ago) was the richest period of oxygen-O2. Reappearance of the O2 provided life with new opportunities, more efficient aerobic metabolism and possibilities. Since the start of the Cambrian period, O2 concentrations have fluctuated between 15% and 35% of atmospheric volume (today 21%). When O2 levels in atmosphere reached 35%, a huge number of insects and amphibians large in size were created.

2.Modern land ecosystems appeared in the Late Devonian, about 385 to 359 million years (the earliest known life on land is 3.48-billion-year-old).

Colonization of the land was difficult because the process of adaptation included: avoiding of drying-out , creation of structures to withstand gravity, development of specified respiration and gas exchange systems have to change, reproductive systems changes (cannot depend on water).

3.The first primates appear around 60-65 million years ago.

One of the first primate-like mammal species is the Plesiadapis from the North America.

.It is probably that these creatures shared the land with the last dinosaurs.

The earliest primates were rat-sized known mainly from teeth, they were insectivorous and nocturnal lived in the trees of tropical forests.

4.Tribe Hominini which includes humans, Australopithecines and chimpanzee parted from the tribe of gorillas around 9 million years ago. Then, the subtribes Hominina (humans and biped ancestors) and chimpanses separated 4–7.5 million years ago.

Evolution of humans is characterized by huge changes in morphology, development, physiology, and behavior. Some of those changes are bipedalism (hands were free for food reaching), changes in structure and brain size (improved social behavior, language, increased inteligance…), and decreased sexual dimorphism.

clumped distribution, because you'de have all of the population gathered in a relatively small are.

it's the most common because it provides protection and a mate for reproduction

example: humans, seeweed, cattles, etc.