"The lapse of time has been so great as to be utterly inappreciable by the human intellect" (Charles Darwin).
Go to more a detailed article on the EVOLUTIONARY SEQUENCE.
Imagine a 450-page book to represent all the ages since the creation of the earth, 4.5 billion years ago . Each side of a page represents 10 million years. If the page has 50 lines, then each line represents 200,000 years and if each line has 50 letters, then each letter represents 4000 years. The front page represents recent time. Our modern western calendar starts with Jesus, half a letter ago compared to the evolution of life !
By 1.8 billion years ago ("page" 180) the first eukaryotic organisms had appeared. The first cells with nuclei (eukaryote) are definitely present in one billion year old sedimentary rocks! Eukaryotes are our ancestors - animals containing cells with a real nucleus enclosed in a nuclear membrane and the genetic material carried on chromosomes.
A nucleus and DNA may have evolved in conjunction with sexual reproduction. A cell nucleus is the great difference between our cells and bacteria. The nucleus enabled the production of more complex proteins than bacteria. It opened the door to great versatility, complexity and plasticity of form. This new feature on the evolutionary scene could be the single cause of the explosion of multicellular forms that occurred! It accompanied a rise in oxygen levels between 2 and 1.5 billion years ago , that was necessary for larger life forms.
By 550 to 540 Mya , (Mya = million years
ago) ("page" 55) oxygen levels had attained a
level near to the current 21 percent of the atmosphere. This level of
was necessary before large, active animals could evolve. Such animals
aerobic respiration and a rich supply of oxygen. Photosynthetic
paved the way for more complex forms of life through their effect upon
the global atmosphere! Every phylum
that exists today, and many modern classes and orders appeared in the
fossil record in a short few million years of the Cambrian explosion.
By 500 million years ago , a strong ozone layer existed, protecting potential emergent plants from lethal short-wave radiation. Land plants evolved from green algae by 450 million years ago (Ordovician period, "page" 45). Terrestrial plants have thus only been around for about one tenth of the age of the earth! By 400 million years ago, spiders, mites, snails, scorpions, centipedes, insects and many other arthropods thrived in large numbers on the land, having followed the diversifying plant forms. The oldest amphibian fossil found to date, Hynerpeton bassetti, is about 365 million years old (late Devonian). It lived in tropical muddy marshland and represented early terrestrial vertebrates. When such animals moved onto land there was plentiful oxygen, insects and plant material. The first reptiles with cleidoic ("closed") eggs emerged about 350 million years ago , but most fossil finds date from about 310 Mya onwards.
From the reptiles of the Carboniferous period , two major lineages and a third minor lineage evolved. Survivors of the minor group are tortoises and turtles. One major lineage is represented today by lizards, snakes and crocodiles. Somewhere and time, from this lineage, birds also evolved. The other major lineage evolved into mammals about 220 million years ago . Flowering plants are a new addition to the earth emerging only 100 million years ago ("page" 10). True placental mammals arose about 130 to 140 million years ago. The world as we know it is very recent.
Primates , the mammal group giving
rise to humanity, appeared fifty to sixty
million years ago . This is only five to six "pages" into a
450-page "book of time" representing the 4.5 billion year age of the
earth! Pongids, the lineage of apes such as chimpanzees, gorillas
and orangutans appeared 20 to 30 million years
ago (Oligocene). Such a 450 page book may have 50 lines
page. Each line then represents 200,000 years, the estimated
period of existence of Homo sapiens sapiens. Early hominids (or
hominid family) (see hominid books)
appeared 7.5 million years
ago or 38 lines into the first page of the book. A hominid (see hominid books) is a member
of the human family, from the earliest ancestral forms that we find in
the fossil record, to modern humans. All hominids share the inherited
character of upright or upright walking on two feet.
Australopithecus is the genus of the most primitive fossils that
are considered a part of the human lineage. Homo erectus
evolved next as a major
representative and then Homo sapiens.
The earliest stone tools appear in the record about 2.5 million years ago or 12 lines into the first page of recent time. Homo habilis made pebble tools about 1.8 million years (nine or ten lines) ago and represents the first Homo species. The "harbinger of humanity" is considered Homo erectus . This creature existed 1.6 to 1.7 million years ago or eight lines into our book of life. Archaic Homo fossils with large brains but with heavy brow ridges like H. erectus appear in the fossil records from about 300,000 years ago. Anatomically modern humans appeared 100,000 years ago.
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