How did our species, Homo sapiens, come to be what it is today, and how are we related to our closest living relative, the chimpanzee? Much has taken place in the past 2.6 million years to shape us into the species we are today.
Last week in Biology Bytes in “Planet of the Apes: Part I,” the evolution of humans from an early mammal to our divergence with chimpanzees was explored. This week we’ll see how humans (members of the genus Homo, which literally means “human being”) have evolved as a group, and how modern humankind has come to be the only living species in this group. This is a particularly challenging story to tell, due both to a lack of intact fossils and to different researchers arriving at different interpretations of what these limited fossils reveal. Consequently, this part of our history contains some of the most hotly debated archeological topics, with multiple theories being proposed that are supported by current evidence. While the most widely supported theories will be discussed, this article will also address some alternative theories that are gaining a following. Ultimately, the new fossil discoveries and new means of examining the evidence will help make clear this tumultuous period of humanity’s evolution.
Becoming human: How did the first members of the genus Homo come to be? “Handy Man” (Homo habilis), whose fossilized remains have been found in several locations in Africa dating to about 1.4 to 1.8 million years ago, is widely believed to be the first member. Handy Man was very different than his likely predecessor (Australopithecus garhi, who roamed Africa about 2.5 to 2.6 million years ago). Handy Man had a much larger brain (averaging 640 cubic centimeters [cc] versus A. garhi’s 450 cc brain) and a more slightly built face than A. garhi, but was overall still relatively small (being roughly chimpanzee-sized). True to his name, remnants show that Handy Man used hammer stones, flakes, and double-edged stones to strip meat from bones and prepare food, creating higher nutritional value. This would have helped him to more efficiently support a brain that required larger amounts of energy, creating an evolutionary positive feedback loop that reinforced further brain development.
Handy Man may have evolved into Homo erectus, though this has recently been doubted: In 2007, in Kenya, Handy Man fossils as recent as 1.44 million years old were discovered, and because Homo erectus lived in the same area for almost half a million years, there may just not have been enough time for Handy Man to have evolved into Homo erectus.
Another contender for the predecessor of Homo erectus is Lake Rudolf Man (Homo rudolfensis), who lived about 1.4 to 1.9 million years ago in Kenya. Lake Rudolf Man may have been a descendent of Flat Face (Kenyanthropus platyops, who lived about 3.2 to 2.5 million years ago, making them contemporaries with the famous “Lucy” [Australopithecus afarensis]). However, the theory that Flat Face and Lake Rudolf Man were predecessors of Homo erectus is very hotly debated. A lack of intact and substantial fossils of Lake Rudolf Man makes it quite difficult to know their proper place in history; most of our knowledge of Lake Rudolf Man comes from one toothless skull lacking a lower jaw bone, and a separate lower jaw bone. Consequently, it has been difficult even to establish just basic facts such as his brain size.
Homo erectus: Overall, there is still much debate over whether Handy Man, Lake Rudolf Man, or perhaps a third Homo species gave rise to Homo erectus. However it came to be, Homo erectus appeared on the evolutionary scene about 1.8 million years ago, and was quite successful. By one million years ago, H. erectus had spread out across Africa, Europe, and Asia. With H. erectus came the emergence of beautifully carved hand axes and a larger brain and body than previous Homo members (around 900 to 1100 cc brains, and individuals standing just over five feet tall and weighing about 120 pounds). They also are thought to have used complex tools, fire, and coordinated groups for hunting, and to have cared for their sick and elderly.
It has long been thought that H. erectus originated in Africa and, from there, migrated out to Eurasia. However, some experts argue that a much older migration out of the continent of members of the Homo genus may have occurred.
One piece of evidence in support of an earlier migration is the “Hobbits” (Homo floreiensis), who were discovered in 2004 on the Indonesian island of Flores by Mike Morwood (University of Wollongong in Australia) and colleagues. The Hobbits, as their name implies, were very small, having a skull similar to H. erectus but much smaller, with a brain size of only about 400 cc, making it close in size to a chimpanzee’s. Standing about three and a half feet tall, and weighing up to 65 pounds, they had a body shape was similar to a much earlier, smaller ancestor. They also used fire, made tools, and butchered carcasses of, for example, the giant rats and pygmy elephants that lived alongside them.
Some evidence suggests people inhabited Flores as early as 800,000 years ago. Taken together, the evidence has led some experts to postulate that the Hobbits may have descended from a much earlier, wide-spread hominid that could have been in the area around 2 million years ago. Amazingly, the Hobbits were living on Flores as recently as 12,000 years ago, making them the last Homo species to become extinct that we know of.
Another piece of evidence in support of H. erectus not being the first humans to leave Africa was found in the country of Georgia, in the town of Dmanisi, where remains 1.8 million years old, of a species called Homo georgicus were found. These people also had relatively small brains (650 cc) but cared for their sick and elderly. Again, this led some experts to postulate that beings before H. erectus, perhaps as long as 3.5 million years ago, spread from Africa to Eurasia.
Of course, not all of these members of the Homo genus were necessarily our predecessors; many most likely belong to branches of our evolutionary tree that became extinct before leaving a group a descendents.
Scattering of Homo erectus: As Homo erectus was becoming established throughout Africa, Europe, and Asia, changes in the global climate could have significantly altered the playing field. A million years ago, 100,000 year cycles in the weather began which may have isolated populations of H. erectus, allowing them to change genetically over time due to their separation. Some groups may have come into contact at later points and interbred or competed, driving some to extinction. While H. erectus became extinct, one emerging survivor was Heidelberg Man.
From Homo erectus to Heidelberg Man: Heidelberg Man (Homo heidelbergensis) first appeared in Ethiopia around 600,000 years ago, and is thought to be a descendent of H. erectus. Compared to H. erectus, Heidelberg Man had a larger brain (1270 cc on average) but a similarly sized body, and ate around 80% plant material, similar to modern hunter and gatherer societies. Heidelberg Man theoretically left Africa for Europe and perhaps China around 500,000 years ago, and may then have given rise to Homo sapiens and Neanderthals (Homo neanderthalensis or Homo sapiens neanderthalensis) before becoming extinct around 300,000 years ago. On the other hand, many believe that Heidelberg Man only gave rise to the Neanderthals, not us; this may change with the recent sequencing of the Neanderthal genome.
The Neanderthals: According to DNA evidence, about 270,000 to 440,000 years ago Neanderthals and Homo sapiens diverged to go their separate ways. Neanderthals were stockier, with deep chests and short fingers (but similar body weights) and were mainly meat eaters; these adaptations may have helped them survive the colder European climate of the time. Their brain sizes were in the range of modern humans (Homo sapiens brains are generally 1300-1500 cc in size, but can vary, while the average Neanderthal brain was 1420 cc). Neanderthals were also pale skinned, with blue or hazel eyes and red or blond hair, and could speak. Additionally, they were quite likely cannibals and performed death rituals.
The first modern Homo sapiens showed up around 200,000 years ago, and around 100,000 years ago started living in caves in the Middle East, specifically Israel. Neanderthals arrived to these caves 20,000 years later, escaping very cold temperatures engulfing Europe. Neanderthals and Homo sapiens may have lived together for as long as 10,000 years. Recently, international research collaborations sequenced the Neanderthal genome (as published in the May 7th, 2010 issue of the journal Science) and it was revealed that most likely during this encounter, 80,000 years ago, that Homo sapiens and Neanderthals interbred. This settled the long-debated question of whether the two groups had sex and could produce children.
Why at this time, and this place? Genome comparisons (specifically between DNA pieces called single-nucleotide polymorphisms) revealed that while Europeans and Asians share 1% to 4% nuclear DNA with Neanderthals, Africans do not share these similarities, suggesting that the interbreeding occurred after Homo sapiens had left Africa, and this was their first stop on the way out. (However, Neanderthals and Homo sapiens might have again lived together in Europe around 30,000 to 45,000 years ago.) Overall, the genomes are 99.4% identical. As researchers discover what genes we do not have in common, we’ll have a better understanding of how we have changed in our most recent evolutionary history. Unfortunately, one third of the Neanderthal genome is still unclear; it was quite difficult to extract intact DNA for analysis, and future efforts may help clear up the picture.
While Neanderthals disappeared about 30,000 years ago, possibly due to competition with Homo sapiens or unfavorable changes in climate, we now know that at least some were absorbed by populations of Homo sapiens and consequently live on in us to this day.
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 the Neanderthal genome in the May 7th, 2010 issue of Science, or Wikipedia’s article on the “Homo genus”.
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 firstname.lastname@example.org.