Ancestor of Humans Lived With Dinosaurs
A very early ancestor to humans originated before dinosaurs went extinct.
An ancestor of humans -- albeit one that is at the root of our family tree -- shared the planet with dinosaurs, a new study concludes.
This ancestor, the first placental mammal, lived between 88.3 to 91.6 million years ago, according to the study, published in the latest issue of Biology Letters. Placental mammals today include humans and all other mammals except those that lay eggs or have pouches (marsupials).
The study counters prior research, based solely on fossil evidence, which theorized this "mother of all placental mammals" arose after the dinosaurs died out. The researchers instead believe that it preceded the non-avian dino die off and that we wouldn't even be here if the dinosaurs were still around.
"When dinosaurs died out, many ecological niches became vacant, and placental mammals took over," lead author Mario dos Reis told Discovery News. "The placental ancestor diversified and evolved into the modern mammals we see today, such as rodents, deer, whales, horses, bats, carnivores, monkeys and ultimately humans."
"If dinosaurs had not died out, then placental mammals may not have had the opportunity to diversify the way they did, and our own species would not have evolved!" added dos Reis, a research associate in the Department of Genetics, Evolution and Environment at University College London.
He and colleagues Philip Donoghue and Ziheng Yang analyzed 36 complete mammal genomes together with information from the mammal fossil record. The results determined placental mammals originated in the Cretaceous.
Dos Reis explained that the DNA of organisms accumulates changes, called mutations, at a constant rate in time. This is referred to as the "molecular clock." For example, certain DNA in humans and other apes mutates at a pace of about 1 percent every 10 million years.
The molecular clock is not perfect, however, and it runs a bit fast in some species and a little slow in others.
Dos Reis and his team therefore "estimated the number of mutations that accumulated in each mammal lineage, corrected for the flaky clock, and together with ages from known fossils estimated the age of the placental ancestor," he said.
Based on earlier research, it's thought that this animal was small, nocturnal and pretty scrappy. It either lived far away from the asteroid impact site that caused the extinction of non-avian dinosaurs, or was somehow saved because of its size, habitat and/or lifestyle.
About 70 percent of all species died out during the mass extinction event 66 million years ago, with even some mammals, birds and plants going extinct then.
"To understand why the big lumbering behemoths went extinct and the gracile birds and mammals did not, we need to further explore the fossil record based on predictions shaped by our molecular analysis which, for instance, suggests the age intervals in which we should find evidence of specific mammal groups," Donoghue told Discovery News.
Michael Benton, a professor in the School of Earth Sciences at the University of Bristol, said he believes that the DNA/molecular clock approach of estimating an animal group's age, used by dos Reis and colleagues, "applies standard, accepted, conservative approaches that take account of missing data in the fossil record."
The first placental mammal might not have looked very human-like, but studies such as this do have important implications for us.
"The relevance to humans is that the placental ancestor is one of our ancestors," Donoghue said.
"It reveals the pattern of assembly of the (basic body plan) that we have inherited. As such, it allows us, for instance, to identify which animals may be best suited to biomedical research to better understand and mitigate congenital diseases."
An artist's rendering shows the first placental mammal.
Back in the Beginning
To put a human face on our ancestors, scientists from the Senckenberg Research Institute used sophisticated methods to form 27 model heads based on tiny bone fragments, teeth and skulls collected from across the globe. The heads are on display for the first time together at the Senckenberg Natural History Museum in Frankfurt, Germany. This model is Sahelanthropus tchadensis, also nicknamed "Toumai," who lived 6.8 million years ago. Parts of its jaw bone and teeth were found nine years ago in the Djurab desert in Chad. It's one of the oldest hominid specimens ever found.
With each new discovery, paleoanthropologists have to rewrite the origins of man's ancestors, adding on new branches and tracking when species split. This model was fashioned from pieces of a skull and jaw found among the remains of 17 pre-humans (nine adults, three adolescents and five children) which were discovered in the Afar Region of Ethiopia in 1975. The ape-man species, Australopithecus afarensis, is believed to have lived 3.2 million years ago. Several more bones from this species have been found in Ethiopia, including the famed "Lucy," a nearly complete A. afarensis skeleton found in Hadar.
Meet "Mrs. Ples," the popular nickname for the most complete skull of an Australopithecus africanus, unearthed in Sterkfontein, South Africa in 1947. It is believed she lived 2.5 million years ago (although the sex of the fossil is not entirely certain). Crystals found on her skull suggest that she died after falling into a chalk pit, which was later filled with sediment. A. africanus has long puzzled scientists because of its massive jaws and teeth, but they now believe the species' skull design was optimal for cracking nuts and seeds.
The skull of this male adult was found on the western shore of Lake Turkana in Kenya in 1985. The shape of the mouth indicates that he had a strong bite and could chew plants. He is believed to have lived in 2.5 million years ago and is classified as Paranthropus aethiopicus. Much is still unknown about this species because so few reamins of P. aethiopicus have been found.
Researchers shaped this skull of "Zinj," found in 1959. The adult male lived 1.8 million years ago in the Olduvai Gorge of Tanzania. His scientific name is Paranthropus boisei, though he was originally called Zinjanthropus boisei -- hence the nickname. First discovered by anthropologist Mary Leakey, the well-preserved cranium has a small brain cavity. He would have eaten seeds, plants and roots which he probably dug with sticks or bones.
This model of a sub-human species -- Homo rudolfensis -- was made from bone fragments found in Koobi Fora, Kenya, in 1972. The adult male is believed to have lived about 1.8 million years ago. He used stone tools and ate meat and plants. H. Rudolfensis' distinctive features include a flatter, broader face and broader postcanine teeth, with more complex crowns and roots. He is also recognized as having a larger cranium than his contemporaries.
The almost perfectly preserved skeleton of the "Turkana Boy" is one of the most spectacular discoveries in paleoanthropology. Judging from his anatomy, scientists believe this Homo ergaster was a tall youth about 13 to 15 years old. According to research, the boy died beside a shallow river delta, where he was covered by alluvial sediments. Comparing the shape of the skull and teeth, H. ergaster had a similiar head structure to the Asian Homo erectus.
This adult male, Homo heidelbergensis, was discovered in in Sima de los Huesos, Spain in 1993. Judging by the skull and cranium, scientists believe he probably died from a massive infection that caused a facial deformation. The model, shown here, does not include the deformity. This species is believed to be an ancestor of Neanderthals, as seen in the shape of his face. "Miquelon," the nickname of "Atapuerca 5", lived about 500,000 to 350,000 years ago and fossils of this species have been found in Italy, France and Greece.
The "Old Man of La Chapelle" was recreated from the skull and jaw of a Homo neanderthalensis male found near La Chapelle-aux-Saints, in France in 1908. He lived 56,000 years ago. His relatively old age, thought to be between 40 to 50 years old, indicates he was well looked after by a clan. The old man's skeleton indicates he suffered from a number of afflictions, including arthritis, and had numerous broken bones. Scientists at first did not realize the age and afflicted state of this specimen when he was first discovered. This led them to incorrectly theorize that male Neanderthals were hunched over when they walked.
The skull and jaw of this female "hobbit" was found in Liang Bua, Flores, Indonesia, in 2003. She was about 1 meter tall (about 3'3") and lived about 18,000 years ago. The discovery of her species, Homo floresiensis, brought into question the belief that Homo sapiens was the only form of mankind for the past 30,000 years. Scientists are still debating whether Homo floresiensis was its own species, or merely a group of diseased modern humans. Evidence is mounting that these small beings were, in fact, a distinct human species.
Bones can only tell us so much. Experts often assume or make educated guesses to fill in the gaps in mankind's family tree, and to develop a sense what our ancestors may have looked like. Judging from skull and mandible fragments found in a cave in Israel in 1969, this young female Homo sapien lived between 100,000 and 90,000 years ago. Her bones indicate she was about 20 years old. Her shattered skull was found among the remains of 20 others in a shallow grave.