Through The Sandglass

Bakken oil: can't anyone rid us of this idiotic viral e-mail?

It's been several months since the last one, so I should have known that it was time for it to come around again. "Fw: Fw: Fwd: FW: Oil - You don't know about‏." The forwarding history and addresses are always interesting - this time sixty or so recipients in total, including a handful of US Senators. The content is always the same ("important and verifiable information"): there are billions - no, sorry, trillions - of barrels of oil under North Dakota and neighbouring states, and it's only an evil (and undoubtedly liberal) conspiracy that's preventing access to US energy independence.

Now, this blog is mainly about sandy topics and explicitly non-controversial. Sand does figure as a bit player in this drama, but sometimes there are topics about which I know a little and that I find so infuriating that, rather than rant at the TV or the computer screen, I will suspend my publishing principles and go to press. This is one of those topics. There is some kind of conspiracy here - one of ignorance, stupidity, and the intellectual feeble-mindedness of the inability, or unwillingness, to question, to check facts.

Almost exactly two years ago, the US Geological Survey (not Service, as the e-mail quotes), published an updated assessment of the potential oil resources of the 350 million year old Bakken Formation in the great pile of sedimentary rocks that is the Williston Basin, long the source of prolific oil and gas production. The USGS follows a rigorous and tested methodology that has been applied around the world to come up with such estimates - estimates being the important term, since these things always come with a health warning about uncertainty - in this particular case, to quote from the USGS Fact Sheet: " there is significant geologic uncertainty in these estimates, which is reflected in the range of estimates for oil." The table of the USGS summary of its results is reproduced below, with the figure of 3.6 billion barrels of oil as the mean estimate highlighted in red.

 

The "F95" and "F5" numbers refer to a 95% and 5% probability that the numbers are equal to, or larger than, those attached to each estimate; the statistical mean is then summarised (mathematically, the means are the only numbers that can be added together and still preserve the statistical validity). Now, the range of such estimates (F95 versus F5) is normally much larger than this, but in this case the small range reflects the peculiarity of the oil resource that is the Bakken; so, a brief explanation of this peculiarity:

1. The Bakken consists of a great pile of shale, extending over a large area and with a sandstone sandwiched in the middle.
2. The shale is formed from marine mud and contains a lot of decayed organic matter.
3. With burial, pressure, temperature, and time, this organic matter has been cooked to the point where it has been transformed into hydrocarbons - the Bakken shale is a great source rock for oil and gas.
4. But a shale, originally mud, is composed mainly of compacted clay, extremely fine-grained rock. The porosity (the empty spaces between the grains that can contain oil,gas, or water), and the permeability (the connectivity of those spaces that allows fluids to flow through the rock) are extremely low.
5. Therefore, while some of the oil generated has escaped and migrated into other parts of the sedimentary pile (including the sandwiched sandstone) and provided the production history of the Williston Basin, the laws of physics mean that much of it remains trapped in the shale.
6. Unlike conventional oil accumulations where the fluids have migrated out of the source rock and into some geological configuration where they are trapped, the entire Bakken shale is an oil resource - as the USGS define it, a "continuous resource."
7. The total volume of the shale and the oil content are relatively straightforward to calculate, and this is why the range of the estimates is not great.

The 3.6 billion barrels are "technically recoverable" by the USGS definition - and here's the critical point. Yes, we have the technology to recover this oil - but at what cost? The USGS estimates explicitly do not address costs or other aspects of economics and commerciality. As others have remarked, yes, we could send a manned mission to Mars - but can we afford it?

Because it's a shale, and its porosity and permeability are so low, the Bakken is extremely unwilling to give up the oil it contains. The required technology involves drilling wells horizontally through the shale and stimulating fracturing of the solid rock, attempting to enhance the natural fractures that increase its permeability. The image at the head of this post (from the full USGS report) shows core samples and microscope images of the Bakken shale - a very solid rock with a few natural fractures along which oil might be induced to flow. All this technology is expensive and the volume of shale around a single well that might be willing to give up its oil is relatively small - a lot of expensive wells are needed and the commercial viability of drilling them is highly sensitive to the price of oil as well as the cooperative (or otherwise) behaviour of the rocks. Today, a number of companies are drilling in the Bakken shale, and production is rising, but another Saudi Arabia it is not.

So where, in the hysterical e-mail, does the figure of 500 billion barrels come from? From an unpublished and un-peer-reviewed 1999 report from a reputable geologist, Leigh Price, who sadly died before the work was completed. His estimate was a mean figure of 413 billion barrels of "potential resources in place." Note the words "in place" - this means the oil that the Bakken potentially contains, not what can be extracted. Those laws of physics, surface tension and so on, that cause oil to stick to the grains of sand in a conventional oil reservoir, mean that recoverable volumes are, even at the best of times, only a fraction of the total of what's there, the in-place oil. This fraction is known as the recovery factor, and in a high quality oil reservoir with great porosity and permeability, a recovery factor of 60% is really good. In a shale, with its miniscule porosity and permeability, even helped by fractures, this number is more like 1%, 5% if you're lucky. So, if you apply these kinds of recovery factors to Price's oil in place, you're in the ballpark of the USGS estimate - for the total resource, not for the economically recoverable volumes.

Where do the trillion barrel numbers come from? Hallucination and distortion on the part of investment snake oil salesmen.

What I have tried to set out here are some facts - that are verifiable by anyone who cares to do so (it's not rocket science). I wrote in my previous post about not taking anything at face value - the proponents of this ignorant and hysterical diatribe clearly have no interest (in spite of the exhortation to "Google it") in following this principle, preferring to believe in a conspiracy: "the environmentalists and others have blocked all efforts to help America become independent of foreign oil!" The e-mail ends with the comment "Now I just wonder what would happen in this country if every one of you sent this to every one in your address book" - it seems as if this has already occurred ; the truth does not emerge from the endless repetition of ignorance.

[A couple of years ago, Brian Romans at Clastic Detritus put up a thoughtful and useful post on this topic, so this is just something of an update. The original USGS Fact Sheet is linked above; their FAQs are also helpful and the full report is available as a somewhat chunky download. There is good commentary at the credible Oil Drum site, Geology.com, FactCheck.org, and, of course, Snopes. Endless entertainment and an astonishing mix of fact and fantasy can be found simply by - as the e-mail exhorts - Googling "Bakken Oil"]

A personal hero

We were baking under the summer sun in a stream bed deep in a mountain valley in Eastern Greece. To the east of us, massive limestone cliffs, to the west, the mountain ranges were built from ophiolitic rocks, the stranded remnants of the oceanic crust that had once formed the great Tethys Ocean. We stood in the geological chaos in between, the tortured testament of the collision of continental fragments, the dismemberment of a continental margin, slope and deeper water sediments shovelled on top of the shelf (the limestone cliffs), the whole pile capped by the ripped up pieces of oceanic crust. The year was 1969, this was my thesis area, and it was my task to unravel the chaos.

This was my first season in the field. I’d succeeded in finding hospitable families in two tiny mountain villages who would put me up, and I had become a popular source of entertainment for the communities. I’d figured out my way around, but regretted that the way out from one village in the morning was invariably downhill, meaning that my return in the evening, tired and with a backpack full of rocks was inevitably uphill. But after an initial period of complete confusion, I felt that things were coming along, that I had a workable scheme of what rocks to map, a sense of where the major faults were (the area had suffered further tectonic contortions even after this bit of Tethys had closed), and generally what I needed to do to beat sense out of this geological mess – I had what I thought was a “working model” (it was only later that I discovered the power of “multiple working hypotheses”).

So there we were, perspiring, scrutinising what I would call a scruffy outcrop in this stream bank. It may have been scruffy – small, weathered, and covered in vegetation, but it was critically important. It was red chert, the siliceous sediment from the deep ocean floor; important because it was uniquely located close to the limestones of the continental shelf, and because the silica that makes up chert originates from radiolaria, tiny planktonic critters that build their skeletons out of silica: they offered a possible means of determining the age of these cherts. Lizards scampered across the rock, crickets chirped, and the sun beat down as I explained my hypothesis of how these sediments got to where they were now.

My companion, to whom I was offering my explanation, was my thesis advisor, Alan Smith, whose invitation to help him unravel the story of this part of Tethys had seduced me back from my post-graduate sojourn in the US, and landed me in the middle of this mess. I have written about Alan before, describing his role in the pioneering “fit” of the circum-Atlantic continents; he had arrived in my mapping area a few days before to see how I was getting on, and it was with a mix of enthusiasm and trepidation that I led him around my crucial locations, my key outcrops. We carefully examined these scruffy red cherts, and then I prepared to move on, believing my hypothesis accepted. But Alan paused, and said to me “Could these cherts be upside down?” “I don’t think so,” was my immediate response, although in truth the possibility had never occurred to me. “Well, can you show me?” was Alan’s simple response. It was, of course, an important question – if the tectonic shovelling had turned these sediments upside down, it would be a key element in the story. We resumed our scrutiny, looking for the small internal features in such sediments that indicate which way was up when they were first deposited – we found none. So off we went, scratching around for hours up and down the stream bed to find even scruffier outcrops that might yield some evidence. Eventually we found the clues – and we both agreed that that these rocks were not upside down. We climbed back to the village and the taverna for a cold beer.

Now this sequence of enquiry may seem quite trivial – but for me it certainly wasn’t. Alan’s apparently simple and innocent question had led me realise that I had made an assumption that fit with my hypothesis but which I had never thought to question. From this simple incident, I learned always to stand back, question my assumptions, ask whether I think I know more than I actually do, and examine all the possible questions that could be asked. The incident, the apparently simple question, has stayed with me ever since – for more than forty years I have tried to take nothing at face value, I have doubted and questioned, and I have – I hope – never run off down the path of an initial model, and an immediate working hypothesis, without stopping to think about what the alternatives could be, however unlikely they may seem. Whenever I’ve reminded Alan of this event, he seems surprised – but he has always been a master at asking those apparently simple questions that ultimately turn out to be the keys that unlock a problem, the ideas that turn conventional wisdom on its head and lead to “breakthrough thinking.” I owe him a huge personal debt of intellectual gratitude. 

[this was written for the latest "Accretionary Wedge" compendium, hosted by Callan Bentley on the theme of Geological Heroes - well worth a visit]

Mark Twain the geologist

 

Mark Twain died one hundred years ago today

“The Old Oolitic Silurian” – my father used to periodically quote this to me to illustrate his familiarity with geological terminology and to remind me of the connections between his own field, American Literature, and my own. Twain was one of my Dad’s favourite writers, and he worked extensively on Twain’s reception in Europe (enthusiastic – Charles Darwin, for example, was a great fan), and his relationships with his English publishers. This led me to appreciate how often, amongst his prolific and highly entertaining observations, Twain wrote about geology – and yes, sand. My Dad’s quotation comes from Life on the Mississippi and ends with one his classic comments:

In the space of one hundred and seventy-six years the Lower Mississippi has shortened itself two hundred and forty-two miles. That is an average of a trifle over one mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oolitic Silurian Period, just a million years ago next November, the Lower Mississippi River was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-rod. And by the same token any person can see that seven hundred and forty-two years from now the Lower Mississippi will be only a mile and three-quarters long, and Cairo and New Orleans will have joined their streets together, and be plodding comfortably along under a single mayor and a mutual board of aldermen. There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.

Twain seemed to have a particular fondness for the Old Red Sandstone, the gigantic piles of sand and other sediments that poured off the growing mountain chains of Europe and North America 400 million years ago. These are the rocks that contain the testaments to the remarkable evolution of fishes and the first footprints of life on the land. Twain wrote the following, in his 1903 essay “Was the World Made for Man?”:

So the Old Silurian seas were opened up to breed the fish in, and at the same time the great work of building Old Red Sandstone mountains eighty thousand feet high to cold-storage their fossils in was begun. This latter was quite indispensable, for there would be no end of failures again, no end of extinctions—millions of them—and it would be cheaper and less trouble to can them in the rocks than to keep tally of them in a book.

Elsewhere, he wrote of “that poor, decrepit, bald-headed, played-out, antediluvian Old Red Sandstone formation which they call the Smithsonian Institute.”

 

Unfortunately, here Twain got his geology wrong – the original Smithsonian building was constructed from the New Red Sandstone, the sediments that filled the rift valleys as Europe and North America began drifting apart 200 million years ago.

Twain also wrote of desert sandstorms and the commercial possibilities of sand. In Tom Sawyer Abroad, Tom, Huck Finn, and Jim take off on a Jules Verne–like voyage in “the boat,” a science-fiction hot air balloon. Sailing over the Sahara, they watch a camel caravan plodding over the dunes below them:

Pretty soon we see something coming that stood up like an amazing wide wall, and reached from the Desert up into the sky and hid the sun, and it was coming like the nation, too. Then a little faint breeze struck us, and then it come harder, and grains of sand begun to sift against our faces and sting like fire, and Tom sung out:

“It’s a sand-storm—turn your backs to it!”

We done it; and in another minute it was blowing a gale, and the sand beat against us by the shovelful, and the air was so thick with it we couldn’t see a thing. In five minutes the boat was level full, and we was setting on the lockers buried up to the chin in sand, and only our heads out and could hardly breathe.

Then the storm thinned, and we see that monstrous wall go a-sailing off across the desert, awful to look at, I tell you. We dug ourselves out and looked down, and where the caravan was before there wasn’t anything but just the sand ocean now, and all still and quiet. All them people and camels was smothered and dead and buried—buried under ten foot of sand, we reckoned, and Tom allowed it might be years before the wind uncovered them, and all that time their friends wouldn’t ever know what become of that caravan.

Realizing that “the boat” (apparently defying the laws of physics) contains several tons of “genuwyne sand from the genuwyne desert of the Sahara,” they consider the commercial potential of putting it into vials and selling it back home (presumably to early arenophiles), “because it’s over four million square miles of sand at ten cents a vial.” But the scale of the operation—and that of the duties they would have to pay—leads them to abandon the idea.

When Twain was 17 or 18 years old, he worked as a part-time typesetter for The Philadelphia Inquirer, still the city’s primary newspaper. Still being marooned in Philadelphia, the Inquirer is my daily paper, and it was their article that drew my attention to the centenary of Twain’s death. He wrote to his brother that “Unlike New York, I like this Philadelphia amazingly, and the people in it.” On the whole, I agree with him – but I do love New York. Reading the newspaper article, I came across a comment that leads to me concluding this with a political viewpoint (something that I generally try to avoid on this blog). The article included the following comment:

Twain is a strange figure to hold up as a national hero. No writer ever sang this country so well – but none ever lanced the boils of its hypocrisy with such needlepoint precision, either.

Isn’t that exactly the kind of national hero every country needs, someone who is equally skilled and perceptive at singing its praises and  lancing its boils?

[Smithsonian image from http://www.american-architecture.info/USA/USA-Washington/DC-019.htm; header photo of the piles of Old Red Sandstone in Greenland courtesy of Dr. Peter Friend] 

Larry's beach - and a scale quiz

Just about exactly a year ago, I had the pleasure of posting some of Larry Deemer’s spectacular photographs from his local beach at Breezy Point, Queens, New York. I’ve kept in touch with Larry since then and, for a couple of bucks and a thirty-five minute subway ride, we escaped Manhattan a couple of weeks ago and journeyed to the limits of Brooklyn. Larry had graciously agreed to my taking up his kind invitation to see his beach for myself, and whisked us down Flatbush Avenue, and across Jamaica Bay – resonant names! – into the borough of Queens  and out onto the ever-changing chunk of Atlantic-facing sand that is the Rockaway Peninsula and Breezy Point. Reflecting its nationally-unique demographics, the community is often referred to as “the Irish Riviera.” Its year-round residents number only a few thousand, amongst whom are our generous and hospitable hosts for that afternoon, Larry, Lou, and Buck. 

What a place to live – just a subway ride and a million miles from Manhattan, with the ability simply to stroll out and enjoy the ever-changing days, seasons, and moods of the beach. It was warm and appropriately breezy afternoon, and we walked, talking, stopping to enjoy and collect the variety of treasures that the beach had to offer that day, watch the birds, photograph the patterns. A truly memorable and immensely enjoyable few hours:

 And here’s a scale quiz: one of the photos below is from Breezy Point beach, the other is from Mars – which is which?

 

Larry told us how the beach had grown over the last fifty years, partly as the result of the construction of the jetty at the end of the sand spit, the longshore drift of sand down the coast from Long Island being backed up behind it, building back eastwards along the coast. So I looked at the USGS website and immediately found two publications on the sedimentary history of Breezy Point and the Gateway National Recreation Area. The links are given at the end of this post, but here’s the introduction to one of them, and an image that dramatically shows the scale of shifting sands over little more than a century:

Had you planned a visit to Breezy Point before the Civil War you would have been in for a surprise. It did not exist! All the land west of the vicinity of Jacob Riis Park has formed within a little more than a century. This is largely because of the construction of a groin field to protect the beaches at Fort Tilden, and the placement of the Breezy Point jetty to prevent the filling of Rockaway Inlet. The sand has been contributed naturally by the westward longshore drift along the south shore of Long Island, particularly the Rockaway barrier island and sources offshore.

 

[USGS publications: http://3dparks.wr.usgs.gov/nyc/parks/loc69.htm; http://pubs.usgs.gov/of/2004/1257/html/gate.htm]

Eyjafjallajokulled - s(tr)anded in the USA

Sitting at the Philadelphia departure gate at 11.00 pm last night on a flight completely packed (presumably as a result of Easter and British Airways strike problems), the pilot informed us that he was "in conference" with Icelandic air traffic authorities as they attempted to maintain the flow of flights across the North Atlantic in the face of the eruption problems. He assured us that, although the ash cloud had attained an altitude of 28,000 feet, he had selected a cruising altitude of 39,000 feet - I'm sure that I was not alone in feeling that this was wise. What he didn't tell us was that the cloud was drifting over much of northern Europe. We became bit-players in the "travel chaos as ash closes air space" melodrama. An hour later, the flight cancelled, we stumbled off the plane into the Philadelphia night, all clutching a small piece of paper with a toll-free phone number, the only assistance that US Airways provided (other than telling us where our bags were - in a completely different terminal approximately a kilometer away).

I was relatively lucky, travelling alone and having my inlaws' home to return to in the middle of the night (and a daughter to pick me up from the station after I'd managed to catch the last train); families of four or five people were stuck with the chaos of trying to find hotel rooms, not to mention the associated costs of a stay of unkown duration. The airline takes no responsibility because they define a volcanic eruption as "weather."

This would, of course, seem a just disruption for a geologist who revels in our planet's activities. I would certainly not want to be in a plane flying into the scene in the photo at the head of this post. And it seems even more an appropriate event for your correspondent, since the ash cloud includes fine sand (indeed, some reports describe particles up to 2 mm, but it's surely not these that are being blown across Europe). As the Guardian website reports, under the unchallengeable title "Volcanic ash: why it's bad for planes,"

Aircraft avoid any airspace that has volcanic ash in it for a simple reason: the ash can wreck the function of propeller or jet aircraft, because it is so fine that it will invade the spaces between rotating machinery and jam it – the silica melts at about 1,100C and fuses on to the turbine blades and nozzle guide vanes (another part of the turbine assembly), which in modern aircraft operate at 1,400C.

That, in turn, can be catastrophic – as the crew of two aircraft, including a British Airways Boeing 747, discovered in 1982 when they flew through an ash cloud from the Galunggung volcano in Indonesia. On both planes, all four engines stopped; they dived from 36,000ft (11km) to 12,000ft before they could restart them and make emergency landings.

That's not the only problem. Ash can pit the windscreens of the pilot's cabin, damage the fuselage and light covers, and even coat the plane so much that it becomes tail-heavy. At runways, ash creates an extra problem because takeoffs and landings will throw it into the air again – where the engines can suck it in and it will create horrific damage to moving parts that suddenly find themselves in contact.

The Icelandic plume has been thrown to between 6km and 11km into the atmosphere – exactly the height that aircraft would be flying.

So, although I'm rebooked on BA friday night, I'm quite content to wait - and may well have to. From USA Today:

The last time Iceland's Eyjafjallajokull volcano blew, the eruption lasted more than a year, from December 1821 until January 1823, reports Sally Sennert, a geologist at the Smithsonian Institution.

"This seems similar to what's happening now," she says.

And it could get worse - historically, eruptions of Eyjafjallajokull have been followed by far more devastating events at nearby Katla volcano - the article goes on to say:

However, as Science Fair noted previously, the Eyjafjallajokull volcano isn't necessarily the main problem. It's Katla, Iceland's noisier neighbor, that's the concern. If lava flowing from Eyjafjallajokull melts the glaciers that hold down the top of Katla, then Katla could blow its top, pumping gigantic amounts of ash into the atmosphere.

The potential eruption of Iceland's volcano Katla could send the world, including the USA, into an extended deep freeze.

"There's no telling how long the eruptions could last," says Sennert about the Eyjafjallajokull volcano."These explosions could go on for some time."

Ah well, I lived in the US for many years and quite enjoyed it......

[image at the head of this post: The Daily Mail/Marco Fuller/Barcroft Media]

An alarming announcement at Barnes and Noble

I have a habit, more often than not masochistic, of dropping into book stores to see whether my efforts are on the shelves. Wandering around the upper Westside of New York a couple of days ago, I spotted a reasonably large Barnes and Noble store and indulged my habit. I was delighted to find two copies of the book on the shelf (the bottom one - the continuing penalty of having a name at the end of the alphabet), but I was also alarmed to find that, next to my book, was a message: "NATURE ENDS HERE."

I recalled the controversial work of Francis Fukuyama from some twenty years ago declaring the end of history, but this seemed far more disastrous. Do Barnes and Noble know something that the rest of us don't? Is this a metaphysical statement placed there by some member of staff embroiled in the angst of a philosophy course at nearby Columbia University? Would there be further clues glued to other shelves in the store? Could it possibly be that some supernatural being uses this communication method instead of tablets and revelations? Is the rapture upon us - but, if so, why would it commence in a New York book store?

I fled in terror but your explanations will be much appreciated.

Footprints in the sand - ancient and modern

 

The great rift valleys of east Africa were and are dynamic places, tectonically and volcanically vigorous as the continent attempted to tear itself apart – and they were also our birthplace. A year ago I wrote about work on the footprints at Ileret, in northern Kenya, that told a small part of the story of our locomotive evolution; recently, there has been a further, fascinating, study reported on the classic footprints at Laetoli, just up the valley in Tanzania. The tracks from Laetoli are far older than those of Ileret, being (probably) Australopithecine prints dating 3.5-3.6 million years ago. Two, and possibly three individuals (one seems to have been walking in the footprints of another) tramped across wet volcanic ash spewed from the Sadiman volcano 20 kilometres away; the ash hardened and this fragment of their journey was preserved, to be discovered by Mary Leakey in 1976. Varied approaches to analysing these prints have been attempted, in the hope of shedding light on how these people (and yes, I feel quite comfortable using that term again) walked, but the recent work of David Raichlen and his colleagues at the University of Arizona at Tucson is particularly appealing. The entire paper is published in PloS One and summarised, for example, on Science News. The paper, titled “Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics,” begins:

Ever since Darwin, bipedal walking has been considered the defining feature of the human lineage. However, how and why this unique form of locomotion evolved remains the subject of considerable debate. In particular, debates over the origins and evolution of bipedalism revolve around whether early bipeds walked with energetically economical human-like extended limb biomechanics, or with more costly ape-like bent-knee, bent-hip (BKBH) kinematics. If early hominins used a BKBH gait, then we must account for the persistence of an energetically costly form of bipedal walking until the evolution of the genus Homo. The Laetoli footprints may help resolve this debate, since they record the footsteps of at least two, and possibly three individuals who walked bipedally across wet ashfall approximately 3.6 million years ago. These prints represent the earliest direct evidence of bipedalism in the fossil record, yet no study to date has demonstrated exactly how these hominins walked.

One of the many reasons that I find this work fascinating is (unsurprisingly) that the researchers used a carefully controlled bed of sand across which modern humans walked, normally and in an imitation of the “BKBH” gait, and sophisticated scanning of the results provided the key means of comparison with the Laetoli prints:

Here, we present the results of the first experimental analysis of footprints in a sample of humans walking with different gaits and compare our results to the Laetoli prints. Eight human subjects (mean body mass [SD] = 65.6 [6.1] kg) walked through a 5 m trackway filled with 15 cm of fine grained sand (0.075 mm–0.70 mm diameter). Although our subjects are likely heavier than the earlier hominins that generated the Laetoli prints, body mass does not appear to have an effect on footprint morphology. We compared footprints made by subjects walking with a normal, extended limb gait, and with a bent-knee, bent-hip (BKBH) ape-like gait at their preferred speeds with sand water content of 6–8% (see supplementary materials for hind limb kinematic data from these experiments). These substrate conditions match those of Laetoli, which are described as similar to damp, fine to medium grained sand . We also examined the effects of increased speed and increased substrate moisture (10–12% water) on footprint morphology. We tested the hypothesis that a BKBH gait alters body weight transfer and produces significantly different footprint morphology than an extended limb gait.

The results are summarised in the figure below, at the top a contour map and side view of the footprint of a modern human walking normally, below it a modern human doing the BKBH  gait (which requires digging in the toes far more deeply), and, at the bottom, one of the Laetoli footprints: the comparisons are dramatic.

 

While it’s clear that these ancestors did not walk precisely as modern humans did, they certainly walked more like us than like apes:

These results provide us with the earliest direct evidence of kinematically human-like bipedalism currently known, and show that extended limb bipedalism evolved long before the appearance of the genus Homo. Since extended-limb bipedalism is more energetically economical than ape-like bipedalism, energy expenditure was likely an important selection pressure on hominin bipeds by 3.6 Ma.

Footprints in the sand – ancient and modern.

[gait illustration used in the image at the head of this post from “Chimpanzee locomotor energetics and the origin
of human bipedalism,” Michael D. Sockol, David A. Raichlen, and Herman Pontzer, available on PNAS]