Mammalian and Human Evolutionary Timeline

I did some work for Peterborough Museum and looked at the evolution of mammals and then humans so I am going to do a post on a timeline of this evolution.

Mammal Evolution:
256 Ma - Wuchiapingian age, Lopingian epoch, late Permian period, Palaeozoic era:

Dimetrodon, an example of a pelycosaur. Image credit
extinctanimals.org

Shortly after the first appearance of reptiles in the Carboniferous period, two evolutionary branches split. The first branch is the Sauropsids, which later become the birds and reptiles of the modern day, the first example of the sauropsid is the Hylonomus. Synapsida is the other branch which gives rise to the mammals.

Both of these branches have temporal fenestrae (openings) behind the orbit which allows for larger jaw muscles. 

The earliest mammal-like reptiles are the pelycosaurs. These animals were the first to have temporal fenestrae. The pelycosaurs gave way to the therapsids, the direct descendants of mammals. The temporal fenestrae of therapsids are larger and more mammal-like than pelycosaurs.  

220 Ma - Norian age, Upper Triassic period, Mesozoic era:
The cynodonts were a subgroup of therapsids and bore the most mammal-like features; it's jaws for example resembled the modern jaws of mammals. It is likely that with in the cynodonts, the direct ancestor to all mammals can be found. 

Artist's impression of Juramaia sinensis alongside
skeleton. Image credit Mark A. Klingler

From Eucynodontia came the first mammals. These were very small, shrew-like animals that fed on a diet of insects. They evolved the neocortex region of the brain and so it is unique to animals.

160 Ma - Oxfordian age, Upper Jurassic period, Mesozoic era:
The earliest known mammal fossil of Juramaia sinensis comes from the Jurassic period, this is the first true mammal that we know of.

100 Ma - Cenomanian age, Upper Cretaceous, Mesozoic era:
The last common ancestor of mice and humans is found here in the Cretaceous period.

Primate Evolution:
85 to 65 Ma - Santonian to Maastrichtian age, Upper Cretaceous, Mesozoic era:
A group of small, insect eating mammals called Euarchonta evolved at the end of the late

Reconstruction of a group of Plesiadapis. Image credit
odec.ca

Cretaceous. This group would give rise to primates, treeshrews and lemurs. Plesiadapis is an animal from the subdivision primatomorpha and lived on the lower branches of trees feeding on fruits and leaves. Plesiadapiformes are very likely to contain the species that are the ancestors of the primates.

63 Ma - Danian age, Palaeocene epoch, Palaeogene period, Cenozoic era:
Primates diverge into strepsirrhini, wet nosed primates, and haplorrhini, dry nosed primates.

Strepsirrhini contains ancestors to lemurs and lorises. The haplorrhines include prosimian tarsiers, simian monkeys and apes. Haplorrhini metabolism lost the ability to make its own Vitamin C, this means that the descendants had to include fruit in their diet. 

30 Ma - Rupelian age, Oligocene epoch, Palaeogene period, Cenozoic era:
Haplorrhini splits into two; platyrrhini and Catarrhini. 

Platyrrhini are new world monkeys that had prehensile tails. It is believed that they migrated to South America, floating on a raft of vegetation is one possible hypothesis for this migration. 

25 Ma - Chattian age, Oligocene epoch, Palaeogene period, Cenozoic era:

Replica skull of Proconsul africanus. Image
credit Don Hitchcock

Catarrhini splits into two groups; old world monkeys, ceropithecoidea, and apes, hominoidea. 

The trichromatic vision had its genetic origins in this period. Proconsul africanus is a possible ancestor of great and lesser apes, including humans.

Hominidae Evolution:
15 Ma - Langhian age, Miocene epoch, Neogene period, Cenozoic era:
Hominidae ancestors speciate from the ancestors of gibbons.

13 Ma - Serravallian age, Miocene epoch, Neogene period, Cenozoic era:
Hominidae ancestors speciate from the ancestor of Orang-Utans. 

The common ancestor of the great apes and humans is believed to be Pierolapithecus catalaunicus. Like humans it had a wide, flat rib cage, a stiff lower spine, flexible wrists and shoulder blades on its back rather than its side. 

10 Ma - Tortonian age, Miocene epoch, Neogene period, Cenozoic era:
The human lineage and the genus of Pan (chimpanzees and bonobos), speciates from the ancestors of gorillas.

7 Ma - Messinian age, Miocene epoch, Neogene period, Cenozoic era:
Hominina speciate from the ancestors of the chimpanzee. 

In the first two years of life, ancestral humans and chimpanzees have a larynx that repositions itself to between the pharynx and lungs; a feature that enabled speech in humans.

3.6 Ma - Piacenzian age, Pliocene epoch, Neogene period, Cenozoic era:
Australopithecus afarensis is evidence for full time bipedalism in early hominids. this ancestor had reduced canines and molars, although still larger than modern humans.

A study of the lower vertebrate of Australopithecus afarensis suggests that in females, changes had been made so that bipedalism could be sustained throughout pregnancy.

3.5 Ma - Piacenzian age, Pliocene epoch, Neogene period, Cenozoic era:
Kenyanthropus platyops is a possible ancestor of Homo, and it emerges from the Australopithecus genus.

3 Ma - Piacenzian age, Pliocene epoch, Neogene period, Cenozoic era:
A loss of body hair takes place in Australopithecines while they evolve on the savannahs of Africa.

Homo Evolution:
2.5 Ma - Gelasian age, Pleistocene epoch, Quaternary Period, Cenozoic era:
Appearance of the genus Homo. Homo habilis and Homo ergaster lived side by side in the lower Pleistocene. The first stone tools were used here.

1.8 Ma - Calabrian age, Pleistocene epoch, Quaternary period, Cenozoic era:
Homo erectus evolves in Africa. Homo erectus resembles more modern day humans, the forehead is less sloping and the teeth are smaller.

Homo georgicus is the oldest hominid fossil outside of Africa, showing that they had the ability to travel long distances, probably following herds of animals.

The evolution of dark skin came with the loss of hair. The brain evolved to be larger and therefore tool crafting was more successful. They could then hunt bigger prey such as wild horses.

1.2 Ma - Calabrian age, Pleistocene epoch, Quaternary period, Cenozoic era:
Homo antecessor may be a common ancestor of humans and Neanderthals. Humans share 99% of their DNA with the now extinct Neanderthals.

600,000 years ago - Middle Pleistocene epoch, Quaternary period, Cenozoic era:
Homo heidelbergensis was found in Italy, it had a larger brain case and was therefore more intelligent than its ancestors, but more muscular than modern humans.

200,000 years ago - Middle Pleistocene epoch, Quaternary period, Cenozoic era:
Earliest fossils of anatomically modern humans found in Ethiopia dating back 0.2 Ma.

60,000 years ago - Upper Pleistocene epoch, Quaternary period, Cenozoic era:
Homo sapiens migrate out of Africa. Homo sapiens interbreed with the Neanderthals that they encounter.

50,000 years ago - Upper Pleistocene epoch, Quaternary period, Cenozoic era: 
Homo sapiens migrate to Southern Asia.

40,000 years ago - Upper Pleistocene epoch, Quaternary period, Cenozoic era:
Homo sapiens migrate to Australia and Europe. The European Homo sapiens known as Cro-Magnon.

25,000 years ago - Upper Pleistocene epoch, Quaternary period, Cenozoic era:
Neanderthal lineage becomes extinct.

20,000 to 10,000 years ago - Upper Pleistocene epoch, Quaternary period, Cenozoic era:
Homo floresiensis dies out, leaving Homo sapiens as the only species of Homo still surviving. Evolution of light coloured skin in Europeans took place around this time.

Evolution - A Brief Explanation

Here I want to go through the basic principles of the Theory, the mechanisms of Evolution and the history of the theory.

In 1859, Charles Darwin published his theory on Evolution. Here he made four key observations. Firstly, organisms produce more offspring than actually survive, for instance for every five offspring born only two will survive to maturity. He also noted that there is variation in the characteristics of members of the same species and that some of these characteristics can be passed on from one generation to the next. Finally, individuals that are best adapted to their environment are more likely to survive. Natural selection is just one process by which evolution occurs. 

Darwin's theory of evolution by natural selection explains his observations. Individuals within a population show variation in their characteristics, this is caused by mutations as a retaliation to predation, disease and competition which creates a struggle for survival. Individuals with better adaptations are more likely to survive, reproduce and pass on their advantageous adaptations to the next generation. Over time, the number of individuals with the advantageous adaptations increases. 

Evolution can lead to speciation, this is where a species evolves into a completely new species. A species is defined as a group of similar organisms that can reproduce to create fertile offspring. Species can exist as one or more populations, for instance there are different populations of American Black Bear in the USA and Canada. Speciation happens when populations of the same species evolve to become so different that they can't breed with members of the unevolved species to produce fertile offspring. 

Sketches of Darwin's Finches, the larger beaks are used for feeding on
 hard nuts whilst the small beaks are best adapted for
feeding on fruit. Sketches are by Charles Darwin himself.

Darwin's finches are the best example for the theory of evolution. The finches of the Galapagos Islands are adapted to feeding on different food sources on the islands. All the species of finch had a common ancestor. Different populations became isolated on different islands, and therefore each population has adapted to the varying environment on the islands. The populations evolved to become so different that they could not interbreed to form fertile offspring and thus created different species of finch. 

There is a lot of evidence to support the theory. Most of this evidence comes from the fossil record. By arranging fossils in chronological order, from oldest to youngest, it is possible to identify gradual changes in organisms that lead to the formation of different species. There is also DNA evidence. Evolution suggests that all organisms evolved from common ancestors, the more closely related two species are, the more recently the species diverged. This is a result of the gradual change in the base sequence in DNA. So, species that diverge more recently will have more similar DNA. A good example of this is that humans and chimpanzees share 94% of their DNA base sequences with each other. 

Different Forms of Evolution:
Convergent Evolution - this is where two species evolve similar features because they are living in the same environment. For example, bats and insects both have wings and can fly but they belong to different phyla.

Coevolution - here two species influence each other's evolution. For instance, flowers will emit a scent whilst being brightly coloured in order to attract insects that then act as pollinators. 

Adaptive Radiation - species rapidly evolve in order to take advantage of ecological niches, Darwin's finches in the Galapagos Islands are the best example here, some had large beaks for eating nuts whilst others used smaller beaks to eat fruits.