Lack of Progress Around “The March of Progress”
Figure 1. Abridged version of The March of Progress.
The image is iconic: A lineup of six figures. At the rear is a chimpanzee-like figure — short, hairy, with a pronounced muzzle, and hunched-over on all fours. The next looks similar but is more upright and walks on two legs. The trend continues, with each successive figure becoming a little taller, a little less hunched, with less body hair and a smaller jaw. Until at the front of the line you reach Homo sapiens. Hairless, fully upright, with close-cropped hair and a neatly trimmed beard, “Modern Man” boldly strides into the future. Instantly recognizable and universally understood to depict the evolution of man, this is one of the most famous scientific illustrations of all time.
It is also wrong.
To be clear, it’s not that evolution is wrong. Humans indeed evolved from primate ancestors over millions of years. It’s this illustration’s depiction of evolution as an unbroken line of progress from ape to Homo sapiens that’s wrong.
The image, originally titled “The Road to Homo Sapiens” though more commonly referred to as “The March of Progress” was created by Rudolph Zallinger for Time-Life Books’ Early Man (1965) volume of the Life Nature Library. It first appeared as a 4.5 page spread of 15 figures; however, when folded to fit within the confines of the book, just six are shown (Figure 1). It’s this abridged version that has exploded into popular culture.
The problem with this abridged version is that it suggests two incorrect facts about evolution. First, by having each figure stand front-to-back in a line it suggests that evolution makes new species by the gradual and complete transformation of one into another. The road to Homo sapiens, in this view, is a straight line from Point A (primate) to Point B (human) with each figure making part of the journey before passing the baton to the next. Second, by having each figure become progressively more human-like and ending with humans ourselves it plays on our biases and suggests that new species are “better” versions of their ancestors . This idea that evolution has an innate directionality, a guiding force steering life towards some goal, is what biologists call “orthogenesis.” Here, the arc of evolution leads from “lower” to “higher,” or “simple” to “complex” organisms, inevitably culminating in Modern Man.
Put these together and evolution becomes synonymous with linear progress: old species transform into better versions of ancestors. The name of the image is, after all, The March of Progress. But these ideas have long been abandoned by biologists, along with the conclusion that evolution equals progress.
(The illustration also oversimplifies other aspects of evolution. By focusing just on recent human evolution it ignores billions of years of evolution acting on other organisms, each with their own unique — and often more interesting — life histories. It also focuses just on changes in physical appearance like posture, height, and body hair, while ignoring the genetic and molecular changes which underly those changes. But even I’m not pedantic enough to complain that one image failed to capture the entire history of all life on earth. Well… not complain too much.)
This might sound like the kind of academic catfight hotly debated in stuffy lecture halls but otherwise of little consequence to anyone outside the ivory tower. But the problem is that this obsolete definition of evolution is by and large its default understanding in pop culture, due in no small part to the ubiquity of The March of Progress.
Just Google “the evolution of” and you’ll find countless modern riffs on The March of Progress. There’s Figure 2, a cartoon in The Denver Post showing the rise (and fall) of communication technologies. Or Figure 3, which shows 50 years of cellphones. Or Figure 4, from Science Magazine, one of the world’s top academic research journals, showing the “evolution” of man to artificial intelligence. More examples have done away with the visuals and made evolution’s very definition a synonym for progress. Each day, companies proclaim their product is “the next stage in the evolution of…” electric cars, or razor blades, or cloud-based blockchain-powered whatevers. The message being: this new thing is better than the last thing. Linear progress.
Unfortunately, this isn’t a new problem. Over 30 years ago, the paleontologist Stephen Jay Gould opened his book Wonderful Life by similarly critiquing several cartoons and advertisements that invoked The March of Progress. If only progress could be made correcting this misinterpretation.
I’ve now ranted at length that about what evolution is not — namely, the wholesale transformation of old species into new, better ones. But I haven’t said what it actually is.
Evolution is a change in heritable traits over time. The raw material of this change is random variation, caused by genetic mutation and recombination. “Speciation,” the formation of a new species, can occur in many different ways and scientists debate the relative importance of each, but most agree that “allopatric” speciation is the most common. Allopatric means “in another place.” It occurs when the geographic range of a species is split by the formation of some physical barrier — the rising of a mountain range or the flooding of a land bridge, for example — which isolates sub-groups of the once connected population. Within each sub-group different mutations will arise independently, and over successive generations this will lead to certain traits becoming more common or more rare. Eventually, so many changes will have accumulated that the two are better regarded different species, than as sub-groups of one species. Evolution occurs not by the sequential transformation of old into new, but by the branching-off of species that co-exist alongside the parental stock. Instead of a linear sequence, multiple speciation events produce a bush with many branches of varying sizes and lengths. Evolutionary sequences like that shown in The March of Progress are an illusion created by tracing the serpentine path from the base of the bush to one twig surviving at the top, ignoring all the side-branches and dead-ends along the way.
This is the answer to the question, “if we evolved from apes why are there still apes?” Though often asked disingenuously, it’s hard to dismiss this question entirely out of hand when the last figure in The March of Progress does look like a lot like a modern chimpanzee. Homo sapiens are apes and we are related to the other apes living today, but we did not evolve from any of them. We evolved from a shared common ancestor. Offspring of that common ancestor diverged and formed new species, which in turn diverged further and formed more species. This process done over millions of years led to modern chimpanzees, gorillas, bonobos, gibbons, orangutans, and — yes — humans.
Evolution is change, and random variation is the raw material of that change, but random variation is not a creative force. It alone could not have built the diversity of life as we know it. Instead, good designs are constructed by “natural selection.” Selection is a process which rejects most of the variation, sparing the few that improve adaptation to the local environment. For example, when you take antibiotics to treat an infection, the drug will kill most of the bacteria — except for any variants that have traits that make them able to resist the drug and survive. These survivors will then reproduce, and in the process pass their genes for antibiotic resistance to the next generation.
Here it’s easy to fall into a trap of saying natural selection makes “better” organisms, so evolution really is progress. But the key qualification is that those adaptations are better only in the context of the organism’s local environment. A trait that’s better in one environment may be worse in another, and natural selection will quickly adjust when conditions change. A fish’s gills are a wonderful adaptation for life underwater, but that advantage is lost if the lake were to dry up. Moreover, natural selection isn’t the only evolutionary process; Chance also plays a key role. For example, if a large population is split and one sub-group is relatively small, it will likely have a non-representative random subsample of the overall genetic variation. Traits which were relatively rare in the original population can either disappear or become much more common simply by chance, in a process called “genetic drift.” Rapid evolutionary change occurs when genetic drift and natural selection act simultaneously on small populations that are forced to adapt to a new environment.
(Evolutionary change also occurs in large populations, but these changes are relatively small, slow to spread, and resisted by a population already well-adapted to its environment.)
Darwin made a point to “never use the words higher or lower.” There is no innate force guiding life up a ladder of complexity towards “better” or “more complex” forms. Apes did not evolve just to be the last stepping stone before humans, just as reptiles did not evolve to turn into mammals, nor fish into reptiles, ad infinitum. Each species is uniquely adapted to their own way of living, shaped by millions years of evolution. Birds can fly from Alaska to Australia, sharks can sense the electrical currents of muscle contractions, thermophilic bacteria live in near-boiling water, and redwoods can grow over 300 ft tall. Who’s to say any of them aren’t more “highly evolved” than humans? “If you judge a fish by its ability to climb a tree it will live its whole life believing it is stupid.”
It’s unfortunate that, while the simplified version of The March of Progress suggests all these obsolete theories of evolution, this was not the illustrator’s intention. The text and timeline that accompany the line-up of figures in the original, full illustration, as shown in Figure 5, present evolution largely in the way accepted by biologists today (though some specific details are now outdated).
For example, not every figure shown is one of man’s ancestors. Ramapithecus (fifth from the left) is described as “the oldest of man’s ancestors in a direct line,” but Oreopithecus, the figure immediately behind Ramapithecus, is described as just a “side branch on man’s family tree,” and A. robustus two figures ahead of Ramapithecus, “represents an evolutionary dead end in man’s ancestry.” Moreover, the timeline above the illustration shows that many of the figures lived at the same time, including the rear and front three in the abridged version. Rather than a linear sequence of one species following the other, the figures in The March of Progress actually represent two bushes of hominids (Dryopithecus, Oreopithecus, and Ramapithecus in one group, and Neanderthal Man, Cro-Magnon Man, and Modern Man in the other) that lived contemporaneously. However, these subtleties are overshadowed by the powerful impression made by the illustration, and completely lost when the illustration is used alone — as is always the case in popular media. Hence, with proliferation of the simplified image came the misinterpretation that evolution equals progress.
So what’s to be done about this? Should scientists ban all use of this erroneous image? In this, I think we should follow the advice of the statistician George E. P. Box, who wrote that, “all models are wrong but some are useful.” Is the simplified March of Progress wrong? Yes. But no scientific illustration, especially not one that can so effectively latch onto the public imagination, is 100% true. This image has established evolution as a fixture pop culture. It’s not an entirely accurate representation of evolution, but it’s closer to the truth than alternative theories on the origins of life on earth. Now established, it’s the job of scientists to steer its interpretation away from linear progress towards how evolution actually works and what it was originally intended to show.
