What Home Looked Like For Seven Million Years

By Carl Zimmer | August 3, 2011 1:01 pm

To understand how we evolved, we have to understand where we evolved. Natural selection exists because the environment is kinder to some individuals than others. Depending on the species, that environment may be a lake miles underneath Antarctic ice, an alpine meadow near the top of a mountain, or an oxygen-free swamp in the sweltering tropics. Each habitat creates its own set of conditions in which individuals thrive or die. We humans are no different. We are the product of where we have lived.

A century ago, paleontologists thought humans evolved in Central Asia. At the time the only known fossils of an ancient human relative (what we now call a hominin) came from Indonesia. The idea of humans evolving in dank rain forests did not appeal to Western scientists who lived in temperate climes. They looked to Central Asia’s windswept plains. In 1926, the American paleontologist Henry Fairfield Osborn laid out this line of thinking in an essay called “Why Central Asia?”

“In that environment, the struggle for existence was severe and evoked all the inventive and resourceful faculties of man,” he wrote. “While the anthropoid apes were luxuriating in the forested lowlands of Asia and Europe, the Dawn Men were evolving in the invigorating atmosphere of the relatively dry uplands.”

It’s hard to imagine worse timing for such a declaration. In 1925, the year before, Raymond Dart discovered the skull of a another hominin in South Africa. It was much older than the one in Indonesia, and it was a lot more ape-like. And since then, paleoanthropologists have found many more fossils of very old hominins in Africa, from South Africa to Kenya and up to Ethiopia and Chad. Hominins first split off from the ancestors of chimpanzees and bonobos (both found only in Africa) about seven million years ago. The oldest hominin fossils date back to about that age, and from seven million to 1.8 million years ago, the fossil record was exclusively African. Only then did hominins start popping up in places like Indonesia and the Caucasus Mountains. Hominins also continued to inhabit Africa, and evolve into new species. The first fossils of Homo sapiens, dating back about 200,000 years ago, are from Ethiopia.

While Osborn was wrong about the place where humans evolved, his vision of the invigorating atmosphere of dry uplands survived. Dart himself wrote in Nature that South Africa offered the same challenging grasslands as Central Asia.

“We must therefore conclude that it was only the enhanced cerebral powers possessed by this group [us] which made their existence possible in this untoward environment,” Dart concluded. For humans to split off from apes, he wrote, they required “a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species.”

Dart offered an early version of what came to be known as the “savanna hypothesis”–that moving from the forests to open grasslands drove the evolution of the hominin lineage, including the evolution of walking upright, a big brain, and even the loss of body hair.

In the decades that followed, scientists tried to reconstruct the ecosystems in which hominins lived. Plant fossils were scarce, but in many cases–particularly in the Rift Valley of East Africa–these studies pointed to open, savanna-like environments.

By the 1990s, however, things started to get confusing. James Shreeve, writing in this 1996 Discover article , explained how paleoanthropologists were finding evidence that the early hominins might actually have lived in more closed woodlands. The debate rolled on, fueled in part by the scarce information scientists had to rely on.

Eighty-six years after Dart first presented the savanna hypothesis in Nature, the journal has now published a spectacular chronicle of the environment in which our ancestors evolved. Thure Cerling of the University of Utah and his colleagues found a way to overcome the scarcity of plant fossils. Plants absorb carbon from the atmosphere to build their wood, stems, and leaves. Carbon in the atmosphere may have different numbers of neutrons–it may be isotopically “heavy” or “light.” Different kinds of plants will end up with a different balance of light and heavy carbon in their tissues. When the plants die, they add their carbon isotopes to the soil. And the soil itself can sometimes turn to rock–a substance known as a paleosol.

Cerling and his colleagues went to African forests and grasslands and measured the carbon isotopes in the soils. They found they could accurately predict the percentage of woody plant cover from the isotopes alone. With this method in hand, they then analyzed 1300 samples of paleosols that formed over the past seven million years from two sites that have yielded some of the richest troves of hominins: the Awash Valley in Ethiopia, and the Omo-Turkana Basin in Kenya. The result is a “tour de force,” according to Harvard paleoanthropologist Daniel Lieberman (who was not involved in the study).

Here is a massive graph summarizing their findings, which will become the benchmark against which future studies of hominin evolution will be measured.

The further to the left the curves are, the more wooded the habitats. The further to the right, the more open they were. In between are bars showing the age of different hominin fossils. The further to the right the bars are, the more small-brained and big toothed they are. The more to the left, the more like us they are. (You can see a bigger version of the graph here.)

Once you’ve grokked this image, join me at the bottom for an explanation of what it all may mean.

Cerling and his colleagues have found that seven million years ago, grasslands with sparse trees existed at these sites. The woody cover increased over the next few million years, reaching its greatest extent about 3.6 million years ago, when the sites were 40-60% woody cover. Then the woods began to retreat. By 1.9 million years ago, there was no place left with more than half woody cover. The environments continued to open up, the trend continuing till today.

I asked Lieberman, who studies the evolution of human anatomy, how he thinks hominin evolution played out against this ecological backdrop.

“Chances are we split from other apes in the forest,” Lieberman told me. He notes that chimpanzees, bonobos, and gorillas all live in the forest today. The fact that we don’t find early forest hominins is probably due to the fact that closed forests are lousy places for fossils to form. It’s probably no coincidence that scientists have found practically hardly any fossils of chimps or gorillas. They’ve lived in the wrong place.

Another crucial fact to consider is that the earliest known hominins have a number of features that hint that they were no longer knuckle walkers. A number of researchers argue that while they couldn’t walk as fast as we can and probably couldn’t run at all, they were already bipedal. So even though the earliest hominin fossils come from lightly wooded East African grasslands, Lieberman suspects that the origin of bipedalism took place earlier, and it took place in forests elsewhere on the continent. (Another paleoanthropologist, John Fleagle, expressed a similar sentiment to me.)

Lieberman suggests that the earliest hominins adapted to the margins of those early forests, where they had to travel further from tree to tree to find fruit. He and his colleagues have found that it’s four times more efficient for a human to walk a given distance than it is for an ape to knucklewalk. Saving energy on these trips could have translated into more babies.

By about seven million years ago, studies like Cerling’s now suggests, hominins were already moving around on two legs through open woodlands. Hominins evolved to be more efficient walkers. They also acquired big teeth and jaws. Lieberman argues that hominins need this new mouth equipment so that they didn’t have to rely on fruit alone. They could also chew on harder, tougher plants like tubers, which served as fall-back foods in the open woodlands.

Although tree cover increased for a couple million years, forests never came to dominate the East African landscape in the past seven million years. And when open grasslands returned with a vengeance, hominins underwent a dramatic change. They got tall and acquired traits that Lieberman argues were adaptations for running. Their teeth and jaws got small; their snouts disappeared.

Lieberman argues that this change marks a new way in which hominins coped with the increasing grasslands: they became hunter-gatherers, traveling long distances to stalk game. And once they began to enjoy this high-protein diet, one more change occurred: the energy-hungry hominin brain was able to expand towards its current size.

But he is quick to point out that there are some important facts about hominin evolution that don’t fit neatly into the scenario he sketched out for me, and a lot of other crucial facts thtat remain to be discovered. “Anybody who isn’t confused doesn’t know what’s going on,” he said. At least scientists now have a better backdrop for finding out exactly what did happen on the way to Homo sapiens.

[Image: Samburu National Reserve in Kenya, Thure Cerling, University of Utah]

CATEGORIZED UNDER: Evolution, The Tangled Bank, Top posts

Comments (28)

  1. re: luxurious vs. invigorating environments, a lot of competition is within the same species for finite resources. in a malthusian system population will grow right up to resource availability, so times of plenty are usually short lived. do animals on the tundra have it easier than animals in a tropical rainforest? if an animal dies in the rainforest is there such a surfeit of resources that it won’t be reprocessed by scavengers? i think this sort of thinking has to reframe these ideas of the all powerful force of environmental pressures. the ecosystem is the environment.

  2. Braggi

    Razib, Malthus’ theory only comes into play post agriculture. He was, basically, an ignoramus anyway. I suggest a quick read of the currently popular “Sex at Dawn” to understand the total collapse of Malthus’ theory. What the author of the article above doesn’t mention is fire. That massive increase in brain size coupled with the changes in tooth, jaw and gut took place once we began cooking our food. It took more than meat to make the change. It took cooked meat and cooked tubers.

  3. Great article up to the second-last graf. Instead of hunter-gatherers and the Pop Ev Psych myths, they became scavenger-gatherers first, opportunistically driving predator animals away from game, in part by using the fire Braggi mentions. It took long after that for Homo to become a hunter-gatherer.

  4. theot58

    This article starts with a loaded question which is inappropriate. “To understand how we evolved, we have to understand where we evolved.”

    This PRESUMES that we did “evolve” – something which is stongly opposed by the scientific evidence. The theory of Darwinian/Macro evolution has many serious problem – to PRESUME that is is true is really poor science.

    The problems for Darwinian/macro evolution include:
    – It does not adequately explain the source information. where did the information contained in the DNA mocule come from
    – It violates the 2nd law of thermodynamics in that it asserts that natural system go from simplicity to complexity, – something that we don not observe.
    – It has no viable explanation of how Sex and genders came into existence

    Prof. Louis Bounoure, Director of Research, National Center of Scientific Research said it well:

    [CZ: Bounoure said no such thing, and never worked at CNRS.]


    animals live in a malthusian world. carry capacity.

  6. Razib, Braggi: Malthus is not really relevant here. The observation that the capacity for animals to breed beyond the limits set by their environment was part of Darwin’s initial postulates. Malthus took this notion of carrying capacity and attempted to apply it to human society in order to prove that poor people will continue to breed until the resources are gone, and therefore that the animal nature of the lower classes makes poverty insurmountable. This load of obvious garbage is what makes Malthus untenable. Carrying capacity is a valid starting point, but it was not Malthus’s idea, nor does it come close to telling the whole story.

  7. This bit about teasing out how much woods there were is really, really cool. It makes you believe that in a few hundred years, someone with a tricorder on some distant planet really could do instant analysis.

    Brain size increased as the woods vanished. So, we’re not saying that causation is proved, only that it looks interesting. I mean, tons of things were happening in the mean time. Socialization might have kicked brain size expansion, aided by sexual selection. Getting good at throwing rocks might have helped kick the brain into planning. Which came first, fire, or a bigger brain? It all probably contributed.

    I’m no Evolution expert. I do stuff with engineering and computers. We sometimes use genetic algorithms to find improved solutions to hairy and ill-defined multi-variable problems. Genetic algorithms are really good at this. You don’t have to tell the computer how to do the search. All you have to do is tell it how good an answer is. It’s amazing how few generations these searches take. So, genetics appears to be very good indeed at searching for new solutions. And death provides a persuasive feedback mechanism. So, uhm, death is a good thing. Who’d a thunk it.

    Information theory does not say that new information can’t be generated.

    As an Engineer, i can vouch for the 2nd Law not having been violated. The Earth isn’t a closed system. There’s plenty of energy from the Sun. High school chemistry shows that more complex stuff can form from simpler stuff. It happens.

    And, as an Engineer, i can vouch for how products evolve. It’s done differently, yes. But engineers don’t design everything, or even very much, from scratch. It’s the search for better solutions, and keeping the stuff that worked. The “life was designed” argument doesn’t hold water because it doesn’t do much to distinguish Evolution from Design.

  8. Jaws13

    @theot58: With due respect, your assertions are incorrect outside of the insular view of the creationist community.
    First, the vast majority of evidence in peer-reviewed journals most definitely supports the evolution of humans and the connection to other primates – if you would like to disagree with the evidence, feel free, but to suggest that the quantity or quality of evidence thus far is on your side is simply false.
    Second, you are correct that to “presume” anything is not scientific. However to make inferences based on sound evidence as these researchers have is absolutely scientific and is done in every field of science, not just biology. So, unless your problem is with science in general, you are barking up the wrong tree there. Adding to the problem is your presumption that we did NOT evolve. Nowhere do you allow for the fact that your position might also be incorrect – this is inconsistent and logically fallacious unless you believe you have an indisputable truth in which case I would love to hear your positive evidence supporting your claims, not simply what you believe to be holes in the opposing view.
    Third, the second law of thermodynamics to the best of my knowledge (I am not a practicing physicist and know that others would explain this better) deals with the flow of energy, specifically heat, over time in an isolated system. Ecosystems are not isolated systems. If you want to claim the entire universe as an isolated system, then order can be maintained by receiving and directing on a consistent basis which life on earth gets from our sun. I’m happy to be corrected on any inaccuracies here, but that’s coming from brighter physics/chemistry minds than mine.
    Fourth, the Dembski et al. “information” claims have been disputed repeatedly and numerous hypotheses exist for the evolution of sex. Again, you can dispute the evidence and claims if you like – such is the nature of science – but then please provide a positive, testable alternative hypothesis. Such is also the nature of science.
    Finally, please don’t dismiss evolution as a fairy tale. You may have good points to make and valid scientific questions to ask – I would surely be happy to hear them and debate them in a reasonable fashion – but to dismiss mountains of evidence and research as nothing more than whimsy and speculation and then posit that the valid alternative is an invisible supernatural force, one that cannot be tested in any scientific way is demeaning to the entire process of science. Ask your questions, poke and prod, but at the end of the day what you cannot test and support with your own evidence is simply faith, not science, and while valid belongs in another discussion.

  9. David B. Benson

    Micht care to combine this with the recent evidence of right-handedness and hence language evolution about two million years ago.

    And when was the ability to control and carry fie thought to have occurred?

  10. Cedric

    The impact on bipedism on human evolution has been very important (allowing more skull space, allowing carrying object about). I have seen some documentaries about how human did not so much evolve to walk but actually evolved to run, using evidence such as the human-specific sweat glands, the lack of hair compared to other mammals of the same size, etc. However I am not sure how widespread this theory is or if it has been discredited?

    What is exciting about all this is to see how theories rise, are struck by new evidence and then we can refine our theories to the evidence. It is sometimes hard for people like me who only reads about it or watches documentaries to really keep track of what are the current theories and which ones have been discredited.

  11. David B. Benson

    Cedric @12 — It is certain that H. sapiens evolved to run; probably for endurance hunting. I’ve not kept up with the details for earlier species, except that Neandthals didn’t run much; this is clear from the diffent cross sections of leg bones in the two species.

  12. saintstephen

    Clear, beautifully written, and completely absorbing. I would be interested in a more detailed explanation of why fossils don’t seem to preserve well in forests where chimpanzees dwell. Perhaps you could address this topic briefly in a future post.

    My thanks for yet another great read, Professor Zimmer!

  13. scarab

    @theot58, post 5 “where did the information contained in the DNA mocule come from”

    To answer that we first have to define information.

    DNA is just sequences of four nucleotides (I’m simplifying). If we extracted the DNA from a cell and left it on a warm bench then the DNA would just: curdle, harden, and generally just sit there. You could watch it for an eternity and it would just sit there, or maybe it would turn to dust and blow away.

    But if you put that DNA in a cell that knew how to read that DNA and could produce proteins from that DNA, proteins that were useful to the cell – then you could see the impact of the information in the DNA. You could, perhaps, add a little jellyfish DNA into a mouse embryo and produce a mouse that glows green. (which has been done)

    So its CONTEXT that matters, the information in DNA is defined by what it DOES for the cells that contain it. That is what determines the information in one arbitrary sequence of DNA from any other arbitrary sequence.

    Furthermore the most useful way of thinking about what a sequence of DNA does for a cell is to consider how it contributes towards the relative reproductive success of the cells that contain it. In other words: its FITNESS.

    Given that we know have a useful way of thinking about the information in DNA (its effects on FITNESS) we are now half way towards seeing how such information can be generated.

    We know that errors occur when a cell duplicates its DNA, there is an extensive literature on the subject: read it. (Google is your friend, try DNA copy errors)

    These errors are random with respect to fitness. Again consult the literature (google for Luria Delbrück). But we can see this intuitively because the changes are a result of ‘mechanical’ errors in the copying machinery.

    What happens when there are small, random changes in a DNA sequence or if fragments of DNA sequences are duplicated?

    The answer is: it depends.

    What does it depend on?

    It depends on the context. It depends on what the changes does to the cells that contain it. It depends on how it affects fitness.

    As a reminder – fitness is how well a given organism does in competing against other organisms – specifically it is its relative success at breeding.

    If a new sequence helps the organism create more viable offspring than other organisms in the population then there is a good chance that the relative proportions of organisms in the population that contain this sequence will rise. This process is called NATURAL SELECTION.

    So we have variation in populations caused by random mutations. These mutations can have different effects on fitness and therefore can be subject to natural selection.

    Natural selection will weed out sequences with less fitness and will increase the proportion of sequences with greater fitness.

    Therefore we have a mechanism that can detect differences in fitness and that can increase fitness in a population.

    Since we have defined information as a contribution towards fitness we can see a mechanism that increases information. That mechanism is random mutation plus natural selection – A.K.A. the theory of evolution.

  14. dave chamberlin

    I love the graph and have to agree that it will become a benchmark against which future hominid evolution will be measured. Savannas are particularly prone to out of control prairie fires during their dry season. One would think the question of when man first made fire would have such an impact as to be recorded in the soil. Several of the above comments mention the undisputed importance of man controlling fire, but from what I have read the timing of this event has only been hypothesized rather than proven. Hopefully an ingenious study such as this one can answer this question in the future.

  15. Andrew

    Fantasic article. I love reading this sort of thing, so thank you.

    Can anyone provide links to information regarding evolving to run (rather than walk) and also regarding the mouse/jellyfish DNA item that Scarab mentions? Thanks!

  16. Mike

    To theot58
    If your only argument for creation is to find the holes in others research you will really need to get that part correct at least. As others have pointed out you got that wrong. Secondly spend some time proposing your own theory, gather your evidence, make sure it explains everything else we see (or at least a great deal of it) and put it on the table for discussion.
    Then, check your assumptions as others with experience in this area will pull it apart before you get to have your opening remarks.
    As for you quote about fairy tales read this. http://www.talkorigins.org/faqs/ce/3/part12.html
    Your re-quoting of re-quoting of what people say is as accurate as your science.

  17. ZD

    lovely stuff, Science is amazing

  18. scarab

    theot58 Says: “It violates the 2nd law of thermodynamics in that it asserts that natural system go from simplicity to complexity, – something that we don not observe.”

    A human starts off life as a single cell. By the time it is an adult it consists of ~100 trillion cells. Google for the number of connections in the human brain then tell us that an adult is not significantly more complex than an egg.

    If you have never noticed that natural systems go from simple to complex then you have simply not been paying attention.

  19. David B. Benson

    saintstephen @14 — Check out the pH of forest soils.

  20. Bipedalism is rightly regarded as key part of human evolution but one aspect of it that is often not pointed out is an important side effect of it. As M. Maurice Abitbol pointed out in “Obstetrics and posture in pelvic anatomy”:



  21. Carolsj

    This makes Terrence McKenna’s theory more likely: that when the apes moved onto the grasslands, they began eating psychedelic mushrooms that grew there, that expanded their consciousness and helped them to evolve quickly.

  22. Theot58, God based biological engineering is so 20th century. Everyone now knows that aliens engeneered us :)

  23. Betty B.

    A very interesting article. Thank you for taking the time to write and publish it!
    “…moving from the forests to open grasslands drove the evolution of the hominin lineage, including the evolution of walking upright, a big brain, …”

    I watched a documentary about a random mutation causing the weakening of the lower jaw, which in turn allowed for the skull to grow bigger, since the jaw bone could no loger pull on the skull plates. That extra skull space allowed for much larger brain size. At about the same time, another random mutation caused our thumbs to develop more detached from the rest of our fingers. This allowed for evolving a grip with much better precision that lead to better tool creation, etc.

  24. Reg Le Sueur

    “theot58 Says:

    – It has no viable explanation of how Sex and genders came into existence”

    It would be charitable to say that this post is just an ignorant rant, but actually in my experience it is deliberate lies and misinformation , because even the terminally ignorant should have managed to catch up with basic science by now:

    1. “information” is a misnomer in this case, because basic information is everywhere and in everything that can behaviour in a logical yes/no mode; eg a light-switch, a mouse-trap, a heat sensor etc. The DNA 4-nucleotide sequence similarly codes for information about which amino-acid to dock with. Nothing mysterious about “information ” here. It is just logic and binary (or triplet) code.

    2. This is the most egregious creationist propaganda of all. The 2nd Law applies to closed systems only. The Earth is an open system; have you not ever noticed the Sun, and how it bombards Earth with photons, driving photosynthesis and all life? Think of a mountain stream running downhill(entropy) ,-driving a waterwheel and doing work and creating order and purpose like being used to grind corn, turn a lathe or grinding wheel, and any number of other applications. This is how order borrows energy from entropy.

    3. Males and females did not appear fully developed out of nothing; they gradually evolved in step with each other, starting from the horizontal gene transfer between primitive living cells by the ingestion of bacteria by bigger Protozoans eg Amoeba, for food. Some of these genes would have survived and become sex cells within the body of its predator , and fused with the predator’s genes, sometimes undergoing reduction division (meiosis), and fusing to form the first zygote (fertisised cell), which could have been “born” as the first individual created by primitive and accidental sex. From then on, it was all evolution until you end up with Barbie Doll and Action Man.

    I think you need to learn some basic science.


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