In the past year, field-changing unveilings on the origins of our species have pushed questions surrounding our evolutionary history into the public spotlight.
What did our last common ancestor with chimpanzees look like, and how did that ape ancestor even evolve to that point? What was our relationship with the extinct, very closely related Neanderthals? The story of our origins, of how the ape called Homo sapiens came to be, was, as much as we don’t like to admit it, largely controlled by chance. Our ancestors often happened to be in the right place at the right time to survive and lead to humankind’s being alive today. Our story begins with a worldwide disaster.
Mammals Taking the Stage: 65 million years ago, the face of the Earth was forever changed as a massive asteroid hit its surface and drastically altered the global environment, coinciding with increased volcanic activity. Essentially all animals living on the land who were larger than the size of a small dog are thought to have gone extinct during the Cretaceous-Tertiary (K-T) event—including, famously, most of the dinosaurs. However, the wake of this devastation opened up many opportunities for the small, shrew-like mammals that survived. Some evidence suggests that there were already a diverse array of large mammals alive, perhaps living as far back as 170 million years ago, but, if they were around, they too met their demise during the K/T event. The small mammalian survivors flourished, filling in leftover niches, and evolved.
Primates Arise: About 60 million years ago, early, squirrel-sized primates arrived on the scene, just in time for the spotlight. Environmental events soon created a world that greatly favored these early primates. About 55 million years ago (between the Paleocene and the Eocene epochs), the Earth became very warm for 100,000 years, heating on a scale that has yet to be repeated. And it happened relatively quickly. In just 10,000 years, the surface of the oceans had increased by about 46 degrees Fahrenheit.
How did this help the early primates? The increased temperatures caused evergreen forests to grow significantly, which increased the available habitat for our early tree-dwelling ancestors, allowing these primates to spread across the world for millions of years.
But it didn’t last. Around 36 million years ago, the Earth became much colder, causing the tropical forests to significantly shrink, and with them the primates’ habitats, as ice sheets began to appear. In North America, for example, the annual average temperature dropped by 54 degrees Fahrenheit. (These changes were caused by much older events: the Indian subcontinent crashed into Asia 54 million years ago, which eventually led to changes in the global atmospheric circulation patterns and, consequently, the observed cooling.) But although their habitat became much reduced and scattered, and many species went extinct, primates still survived this prolonged period of global cooling, specifically in suitable habitats near the equator.
The Coming of the Apes: Fossils of the first ape-like creatures date back to about 23 million years ago, at the start of the Miocene. These animals, too, made an appearance just at the right moment on the world’s stage: From about 23 to 15 million years ago, the Earth warmed again temporarily, causing tropical and subtropical forests to expand in Africa, and some across Eurasia. More forests meant the burgeoning apes had a wider food selection, expanding their diet and range.
However, during this period, Africa became separated from Eurasia by a seaway, leaving the only living apes stranded in Africa. Near the end of this period the continents had an intermittent land bridge connecting them, until they became permanently connected 14 million years ago. Some apes traveled into Eurasia while this bridge was temporarily present, becoming separated from their relatives, and diversifying in the new forests. Consequently, by nine million years ago, a variety of apes lived throughout evergreen wooded environments in northern Africa and southern Eurasia.
However, the environment once again let it be known that it, ultimately, dictates the fate of these creatures and all life on Earth; around nine million years ago, woodlands disappeared in northern areas, leaving apes living only in parts of southeast Asia and tropical Africa.
Diverging from Apes: Based on fossils and DNA data (much elucidated by Morris Goodman and colleagues at the Wayne State University School of Medicine in Michigan) we have an idea of when the lineage of apes that would eventually give rise to us branched off from our other ape cousins. Time ranges given vary greatly between studies, however, only allowing for rough estimations.
Orangutans were the first apes to go their separate way from our ancestors; our last common ancestor with orangutans lived over nine million years ago. Around eight million years ago, gorillas and our ancestors also parted. The last existing ape we shared a common ancestor with is the chimpanzee; our common ancestor is thought to have walked the earth around 5 million years ago.
Diverging from Chimps: This brings us up to a finding that has been all over the news in the past year; our possible ancestor Ardipithecus armidus, or just “Ardi.” In 2009, 11 papers were published in the October issue of the journal Science on Ardi, although he was first unearthed in 1992. It took the discoverers, Tim White at the University of California, Berkeley, and collaborators, several years to carefully excavate the crushed remains of Ardi, and then several more to digitally “rebuild” the skeleton using virtual models. But the results have been worth the long wait. Ardi’s fossils form one of the most complete skeletons of the early hominids. Plus, Ardi is possibly the oldest bipedal human ancestor that has been discovered, and the last known common ancestor with chimpanzees, though both of these claims are quite hotly debated by experts.
Around 4.3 to 4.5 million years ago, Ardi’s species is thought to have roamed the area that is now Ethiopia. The area was an open, grassy, bushy woodlands at that time; Ardi had ventured into a much more open space than his forest-dwelling predecessors had preferred. Ardi is thought to have been able to do an “intermediate” form of bipedal walking: In contrast to other apes, it does not look like Ardi knuckle-walked or swing on branches. It does look like Ardi could still climb trees well, though. This bipedal characteristic (though much debated) supports the idea that walking upright evolved before our ancestors lived in grasslands, while they still lived in a wooded area.
If in fact Ardi’s species is our last common ancestor with chimpanzees, he nevertheless did not look like a transition between chimps and humans; chimpanzees clearly went their own way after this divide. Standing nearly four feet tall and weighing 110 pounds, Ardi was about the size of a chimpanzee, with a similarly sized brain. Ardi was more omnivorous than chimps, though, and may have walked upright more often than chimps do.
However, Ardi’s position in the ape evolutionary tree is much debated. Some evidence suggests that Ardi arrived too late to be the common ancestor of chimps and humans. Others postulate that Ardi’s branch did not give rise to our ancestors, but was a side-branch that became extinct. Clearly much remains to be determined. Further studies of Ardi’s skeleton, as well as the skeletal remains from at least 35 other individuals of Ardi’s species that have been found, should help clarify our relationship to Ardi, and his to chimpanzees.
From Ardi to Lucy? If Ardi indeed turns out to be our ancestor, it is likely that he gave rise to a later hominid, called Lake Man (Australopithecus anamensis), an upright-walking ancestor who ventured into open, dry woodlands and bushy savannahs near fresh water in the area that is now Ethiopia and Kenya, around 4.2 to 3.9 million years ago. Lake Man, in turn, is widely thought to be the ancestor of the famous Lucy (Australopithecus afarensis). Lucy was discovered in 1974 by Dr. Donald Johanson while he was working for the Case Western Reserve University in Ohio.
Lucy, who lived about 3.0 to 3.9 million years ago, also showed us that upright walking came before the big brain. Lucy lived in even more open habitats than any hominid had before, in open savannahs and bush lands, and became widely present in Africa. However, experts even sometimes question Lucy’s claim of being our direct ancestor; she too could have been an off-branch species that became extinct, while our ancestors had already branched off. Quite possibly, there were several “trial” hominids that became extinct, while only our immediate ancestors survived to give rise to Homo sapiens.
From Lucy to Humans: Climate change once again set the stage. As Earth cooled and dried 3.5 million years ago, forests became broken up. Open vegetation areas became a dominant feature of the African landscape by two million years ago. Many species again went extinct. But around 2.6 million years ago, an amazing thing happened: An ape made a tool out of stone. Some of these small-brained protohumans would eventually lead to us.
However, much occurred in those 2.6 million years to get us where we are today; and that “recent” evolutionary story will be the focus for next week’s article, in “Planet of the Apes: Part II.”
For more on the evolution of apes, see the amazing book The Humans Who Went Extinct, by Clive Finlayson; the book The Last Human: A Guide to Twenty-Two Species of Extinct Humans, by G. J. Sawyer and Viktor Deak; articles on Ardi in the October 2nd, 2009 or November 18th, 2009 issues of Science, or Wikipedia’s article on
“Primates.”
Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at science@independent.com.