Humans are the only surviving branch of a diverse family tree. Follow the evolutionary journeys of our ancestors.
Lineages ~ Continue the Documentary
EXTINCTION IS A NATURAL PART of the evolutionary process. Most species that have ever existed on this planet are extinct, including all our hominid ancestors. A healthy ecosystem balances species formation and extinction; thus the extinction of the dinosaurs created opportunities for our mammalian ancestors to thrive. Will our species also go extinct? It is impossible to say.
Ecologists fear that extinctions today are happening too quickly; human interventions have put the ecosystem out of balance. The vast human population threatens natural habitats & endangers thousands of interdependent species. Because humans have already caused numerous extinctions by overhunting, habitat destruction, & many other acts, we must at least become better custodians of the earth.
Click for > Hominid Profiles
Animation based on reference art by Cathy Willermet
Wolpoff photograph courtesy of University of Michigan Photo Services
Rak photography courtesy of Donald Johanson
SCIENTISTS TRACE HOMINOID fossils back to the late Oligocene (around 27 million years ago) and the Miocene (23 million to 5 million years ago). These fossil apes evolved the traits we recognize in apes and humans today: long arms, lots of joint mobility for hanging below tree branches, the Y-5 molar, and the lack of a tail.
The beginning of the Miocene was a good time to be an ape. The world was warming after a bitter ice age, and thick rainforests abounded. Among the great diversity of life at this time, ape species vastly outnumbered monkeys.
By the end of the Miocene, around 10 million to 5 million years ago, the world had become much drier and colder, and many of the rainforests had become open woodlands. Most ape species all over the world went extinct. One survivor was the ancestor of the orangutans in Asia. Another was the common ancestor of African apes and humans, which lived between 8 and 5 million years ago.
Hominid fossils from the species Ardipithecus ramidus and Australopithecus anamensis, both over 4 million years old, have recently been discovered. These exciting finds move us closer to identifying the common ancestor to African apes and humans.
SCIENTISTS CAN MEASURE how closely two species are related by comparing their DNA. All life on this planet shares the same DNA bases. Differences in the base order cause differences in form. Some genes, or traits, control the function of other genes; a single change in these regulatory genes can produce a significant effect.
Species that are similar genetically share an ancestor in the not too distant past. When scientists compare chimpanzee and human DNA sequences, 98% of the sequences match and 2% are different. That 2% makes a big difference, but it also means these two species have a very recent common ancestor.
did this common ancestor live?
Fossil analysis helps calibrate the molecular clock by placing the common ancestors in time. Scientists believe that the common ancestor of humans, African apes, and orangutans lived between 16 and 10 million years ago. By measuring the amount of DNA difference between humans and orangutans over those 6 million years, scientists can derive a mutation rate. They use that mutation rate, in turn, to estimate the time needed to build up the differences between African apes and humans. That calculation places the common ancestor of African apes and humans between 8 and 5 million years ago.
We can estimate how long ago two species shared a common ancestor using a molecular clock.
Random changes in the DNA code, called mutations, accumulate in all species over time. Some researchers believe that some mutations are selectively neutral, making the mutation rate within lineages roughly constant over time. Comparisons of the DNA of orangutans, African apes, and humans reveal that African apes and humans are much more similar to each other than either is to the orangutan. Scientists hypothesize that African apes and humans shared a common ancestor with each other much more recently than they did with orangutans. But how long ago
Out of Africa
In the multiregional theory, human populations in regional areas slowly evolved into modern humans.Modern traits arose in some populations and spread to others through geneflow. Interbreeding was possible between any two populations, including Neandertals and modern humans.
The "Out of Africa"theory proposes that modern humans evolved from a local population of archaic humans in Africa between 200,000 and 100,000 years ago.This group migrated from Africa and eventually replaced all the other human populations in the world. Only one population of archaic humans is our ancestor.
Africa was the sole setting of human evolution for the first three or four million years of hominid existence. The first species to begin a spread into new continents was Homo erectus. There are different theories about how our species, Homo sapiens, spread across the globe. Did all modern humans come from a single African population, or did modern humans arise from different regional populations?