24 MILLION YEARS AGO

Even though it may have started as early as 25 to 28 million years ago, the start of the Miocene epoch is marked by the development of the East African Rift (EAR) — an active continental rift zone designed to split the African tectonic plate into two called the Somali Plate and the Nubian Plate. Measurements in modern times has revealed a separation rate of 6 to 7 millimetres per year. As the tectonic plates separate, two mountain ranges were formed by around 8 million years ago. The consequence of this geological activity is that rainfall would decrease east of the continental rift zone, leading to fewer trees. Despite this change, it would provide a motivation for certain primates to adapt to the new environment and see the advantage in walking upright on two feet on the ground when moving from tree to tree in search of food.

18 to 22 MILLION YEARS AGO

Prior to 22 million years ago, Greenland had a plentiful supply of pine trees carpeting the valleys in the south. However, during this period, the landscape would change. Regular snow in the Arctic region and shorter summer periods would eventually form solid ice that would not melt, and eventually glaciers would grow across valleys, thereby putting an end to advanced plant life on this continent. The only time the ice stranglehold on Greenland would diminish and eventually break is in the latter half of the 21st century as greenhouses gases in the atmosphere from melting permafrosts and man-made activities (such as mining and burning oil) warm the entire planet.

17 to 14.5 MILLION YEARS AGO

Between 17 and 14.5 million years ago, Earth experienced the Middle Miocene Climatic Optimum (MMCO), a warm interval when global temperatures rose and tropical forests expanded. This period supported the diversification of many mammal groups, including early apes, which flourished in Africa and Eurasia. The MMCO ended abruptly as global cooling intensified, leading to the expansion of Antarctic ice sheets.

14 to 15 MILLION YEARS AGO

During a major global cooling event known as the Middle Miocene Climate Transition (MMCT), ice sheets across Antarctica expanded to cover much of the continent. This expansion lowered global sea levels significantly and altered ocean circulation patterns, influencing marine ecosystems worldwide.

20 to 12 MILLION YEARS AGO

The Miocene was a golden age for ape evolution. Between 20 and 12 million years ago, dozens of ape species spread across Africa, Europe, and Asia. Genera such as Proconsul, Morotopithecus, Pierolapithecus, and Dryopithecus represent early experiments in body form, locomotion, and diet. This period saw the emergence of traits such as flexible shoulders, upright posture, and varied feeding strategies that would later influence hominid evolution.

13 to 10 MILLION YEARS AGO

Temperatures dropped further and substantially across the globe. Large glaciers are a permanent feature of the Antarctic continent, arctic North America, and other similar areas. Whether this cold period is the result of (i) renewed mountain building; (ii) sudden plant growth throughout the world; (iii) a reduction in energy output from the Sun; or (iv) the possibility that the Earth is wobbling along its orbit after an asteroid had impacted with it (or a massive volcano had erupted), scientists can only speculate at this time.

The great whale predators were among the 20 per cent of animals to become extinct at the time the first glaciers appeared on Antarctica.

10 to 5 MILLION YEARS AGO

Nearly 10 million years ago, central Africa contained a vast and thick forest considered bigger than what we see today in the Congo region. However, as time passed and the commencement of a glacial period in the late Miocene as well as mountain building in eastern Africa, the forests gave way to fewer trees and more open grassland plains throughout much of the African continent. In northern Africa, tree coverage would be fewer and far between due to the increasing (but not yet significant) rain shadow effect brought on by the Himalayan mountain range. Still, there was opportunity for those willing to take on the advantages of a changing African continent. In fact, the environmental change would prove essential for our earliest ancestors to evolve further by starting to come down from the trees more regularly and learned to adapt to the harsher, more open landscape. A necessary adaptation in order to find more food and to keep away from the extra competition from our chimpanzee cousins by finding unoccupied trees. It was also a good time for our ancestors to learn to stand up and observe the surroundings for predators and, in some cases, discover the joys of balancing on two feet and walk, and then run when a predator is sighted.

Well, it turned out this was the case between 6 and 7 million years ago when the lineage leading to modern humans broke away from the chimpanzees — our oldest surviving relatives. At the time the break away species appeared, we relied heavily on the trees for protection and to sleep. But as the trees began to thin out, we got bolder and riskier in our attempts to move from tree to tree and gathering food on the ground by walking across the ground.

9 to 5 MILLION YEARS AGO

As climates cooled and grasslands expanded, many forest‑adapted mammals declined while grazing species such as horses, antelopes, and elephants diversified. This faunal turnover reshaped ecosystems across Africa and Eurasia and set the stage for the environments encountered by the earliest hominids.

6 MILLION YEARS AGO

In September 2004, a French-led team of scientists studied part of the left thighbone of a chimp-sized human-like hominid that lived in Kenya on the great African continent over 6 million years ago. Dubbed the Millenium Ancestor (or the more scientifically-correct term of Orrotin tugenensis), this hominid has been found to have the ability to walk upright on two legs for a substantial part of its life. This creature is certainly considered by scientists to be our ancestor, and for good reasons too.

Originally found by French scientists in 2000, the bones of Orrotin tugenensis were independently analysed by American scientists to help determine the age of the bones and whether the creature could have walked upright by measuring the bone thickness in the upper and lower portion of the thigh bone. Now the results are in and the facts are truly amazing. The thickness of the bone in the part that meets the ball in the thigh was said to be thinner than the bone meeting the knee joint by a ratio of 1 to 3 according to CAT scans. For modern humans, the ratio is at least 1 to 4. For modern chimpanzees and gorillas that still rely on walking mostly on four legs, the ratio is approximately equal.

The conclusion is virtually undeniable: this 6 million year old hominid could and probably did walk upright for much of its life.

As Dr Robert Eckhardt of Pennsylvannia State University, which led the CAT scan team, said:

"In present-day chimps and gorillas, the thickness in the upper and lower parts of that bone are approximately equal. In modern humans the bone on top is thinner than on the bottom by a ratio of one to four or more. The ratio in this fossil is one to three. We have solid evidence of the earliest upright posture and bipedalism securely dated to 6 million years." (1)

UPDATE
2002

Scientists have found the remains of a 7-million-year-old skull named "Toumai" in the central African country of Chad. What has excited the scientists with this find is in the shape of the teeth and jaw — it looks remarkably human. Yet the rest of the skull looked nothing more than those of an oversized chimpanzee. This is the oldest skull to have this human/chimpanzee combination feature, and some scientists are claiming it could be a direct link to our earliest known ancestors.

UPDATE
April 2005

Scientists have officially confirmed with the help of computer reconstruction of the skull and jaw that the creature named "Toumai" is indeed more human than ape. This means the skull could be our oldest known ancestor. And it is possible this creature may have walked upright.

5.96 to 5.33 MILLION YEARS AGO

Near the end of the Miocene, the Mediterranean Sea nearly dried out during the Messinian Salinity Crisis. Restricted water flow from the Atlantic caused the basin to evaporate repeatedly, leaving behind vast salt deposits and dramatically altering regional climates. The crisis ended when the Atlantic refilled the basin in a catastrophic flood at the start of the Pliocene.

4.4 MILLION YEARS AGO

During this time, one species of primate appeared known as Ardipithecus Ramidus (not the same as Ramapithecus, which is now known to be an early ape, not a human ancestor). This 50kg primate lived in the woodland environments of what is now Ethiopia. Unlike earlier apes, Ardipithecus had reduced canine teeth and a pelvis and foot structure suggesting a form of upright walking, although it still climbed trees extensively. Fossil evidence shows that it inhabited a forested landscape rather than open grasslands, indicating that early bipedalism evolved in wooded habitats rather than in response to savanna conditions. Ardipithecus is considered a likely early hominin.

Indeed, it is likely Ardipithecus was a slightly more advanced version of Orrotin tugenensis, having evolved to be a taller species (even though it probably stood not much more than 120cm) capable of surviving longer by standing up and surveying the countryside at ground level much better in order to identify predators and to find food, even though it still relied on trees for shelter and protection.

Another clue to the human connection can be found in the thigh bone of Ramapithecus. It was found to be midway between a chimpanzee and a human. In other words, the thigh bone was getting wider and shorter compared to the narrower and longer thigh bone of chimpanzees.

NOTE: With so many prehistoric skulls of the hominid variety uncovered in Africa, the continent has been described as the cradle of mankind. DNA evidence would later support this picture.