The study of form has been central to biology ever since people have contemplated how life came to exist and how individual species or groups of them are related to one another.
When biologists speak of ‘form’ they mean the shape, appearance or structure an organism takes — be it whole organism or only a constituent part such as a bodily system, organ, microscopic structure or even a molecule.
A famous example from the 20th century is the form of the DNA molecule, which we have known to be a double helix since Watson and Crick published their model in 1953.
Palaeontologists like me are especially interested in form because it gives us clues about the diversity of past life and deep insights into the history and mechanisms of evolution.
Most organisms seem to be well designed for their ecological circumstances, an observation that is as old as biology itself.
Why this is the case and how well the fit between organism and environment actually is remain fundamental questions still in evolutionary science today.
Developing views about form
Interest in form goes back to the Ancient Greeks who were the first people to formally observe the great variety of life and explain how it came into being.
In his work Historia Animalium (The History of the Animals), Aristotle (384-322 BC) produced one of the first scholarly works devoted to the subject of comparative anatomy, or the comparison form among animals.
A little later, the Roman naturalist and philosopher Pliny the Elder (about 23-79 AD) also pondered the diversity and relationships of life and was also the first person to describe the strong resemblance of humans to primates in his book Naturalis Historia (Natural History).
Galen of Pergamon (c130-199/ or 217 AD), a prominent Greek physician, surgeon and philosopher in the Roman Empire, and whose ideas dominated Western medicine for over 1000 years, also recognised similarities in form between humans and primates.
He is even said to have commended his students to study primate anatomy in order to develop a better understanding of humans.
Seventeen centuries later, Charles Darwin provided in 1859 a mechanism by which favourable forms could become widespread within species and persist over long periods, namely, through natural selection.
His understanding of how form arose within individuals, and was inherited and modified, remained rather rudimentary, and his speculations were ultimately shown to be incorrect.
These problems would have to await a proper understanding of growth and development, including embryology, and the principles of inheritance, all of which were developed during or soon after Darwin’s time.
It was really with D’Arcy Thompson’s 1917 book On Growth and Form that the examination of form took on a more scientific bent.
While others before him like Goethe had recognised the importance of form — inventing terms like ‘morphology’ to describe it — theirs was a largely descriptive approach, not an explicitly geometric or mathematical one.
In 1968, in his book Order and Life, James Needham wrote, “the central problem of biology is the form problem,” and it remains so today.
In the 1990s the scientific discipline of ‘evo-devo’, or evolutionary developmental biology, blending embryology, genetics and evolution, began to provide deep insights into how form was constructed and how it changed.
Evo-devo marked the beginning of a profound shift in our understanding, but there remain many unanswered questions.
A limit to form?
One question that has plagued the study of form since before Thompson is whether nature sets limits on how many different forms life can take.
Or put another way, is evolution constrained in the solutions it can invent to solve the ecological problems species face?
Influential palaeontologists like Simon Conway-Morris of Cambridge University, for example, who has devoted his career to studying the Cambrian explosion of animal life argues there is indeed a limitation to the number of possible forms life can take.
The evidence for this, he argues, comes from the prevalence of repeated forms in nature across distinct evolutionary lines, or what biologists call evolutionary convergence.
If this were correct, what might be the cause of such limitations to form? Is there, for example, only a limited number of combinations that genes can take?
The genes that control the body plan in the developing embryo such as the Homeobox cluster are after all highly conserved and very ancient.
There are striking similarities in these genes even between humans and fruit flies.
But Conway-Morris sees repeating forms in nature as evidence for God or design.
For me, Creation is too complex a solution to be satisfactory, one that raises far more questions that it answers, and one not really amenable to testing within the scientific framework.
Besides, evo-devo has opened up many more possibilities now including tinkering by natural selection in the timing of key events during embryogenesis through genetic mutation or even epigenetic influences.
Deep insights from apes
If we take a close look at the animals I’m most familiar with — humans and our ape cousins — we find good reasons to be sceptical that the kind of constraints on form that Conway-Morris supposes actually exist.
Research published recently by Sergio Almécija of George Washington University and co-authors shows that scientists have dramatically underestimated the ways in which the body form of apes can and do vary.
For primates, whose lives are spent mostly or entirely within trees, grasping hands are one of the main ways in which they interact with their environment.
Hands hang onto branches when primates moved about or rest, they grasp food for eating, and apes groom each other using their well developed handgrips.
Almécija and colleagues examined the proportions of the bones of the hand and hand digits in humans and other apes and found that they varied an awful lot, much more so than we had all been led to believe before now.
They found that among the apes, gibbons possess a highly unique form of hand, while chimpanzees and orangutans had similar hands, which had evolved independently of one other.
Gorillas and humans had very conservative or ‘primitive’ hands that had changed very little during our evolutionary histories.
Even today, human and gorilla hands look an awful lot like monkey hands rather than those of our chimpanzee cousins.
So our monkey-like human hands also turn out to be an awful lot like those of the earliest members of our evolutionary group, the bipeds who first strode the African savannah seven million years ago.
One big implication of this work is that it challenges a long-held assumption that living chimpanzees are a lot like our earliest human ancestors: a kind of evolutionary snap shot of our own earliest bipedal ancestors if you like.
For decades now we have been studying chimpanzees to glean insights into our immediate evolution, but this approach looks increasingly problematic.
Coming back to where we started, there seem also to be far fewer limits to form than many scientists have believed, and that important features like hands can change or stay the same in ways that are not immediately obvious or predictable.