See more in our gallery: “Human and animal freaks of nature“

THERE’S no getting around it: freaks are fascinating. We no longer pay to gawp at bearded ladies in sideshows, but turn on the TV and you will find such documentaries such as The woman with half a body, The boys joined at the head or The girl with eight limbs (all real titles). For many of us, it’s a morbid curiosity, but Mark Blumberg wants to harness this fascination to a noble end. Understanding how freakish developmental anomalies come about, he argues, can help us understand how normal bodies are made and how evolution has produced such a dazzling diversity of forms.

Many biologists automatically ascribe freak qualities to mutated genes, but some are realising that embryonic development can play just as important a role. Conditions like cyclopia (one central eye) and dicephalus (two heads on one body) do not require genetic mutations but can occur if, for instance, the mother drinks alcohol while pregnant.

The key, says Blumberg, is that development is not a rigid programme. Instead, the embryo monitors its environment and adjusts accordingly. There’s a clear evolutionary advantage to this. Imagine navigating a road trip using a specific route you downloaded off the web. The list of instructions is fine – until you get lost or hit a traffic jam, at which point you’re stuck. A map, on the other hand, would allow you to find alternative paths to your destination.

The journey from fertilised egg to trillion-celled adult is so complicated and chancy that none of us would make it if there were no room for last-minute manoeuvres. So development is like a map with the destination circled: responding to internal and external factors, the body finds its way one step at a time.

It’s a testament to the system’s robustness that even when things go very wrong – if two embryos fuse into a conjoined twin, or if thalidomide distorts limb growth – the resulting beings can survive and even thrive. Bones and muscles can reshape so that, for example, a goat born without front legs can walk upright. Not only that, brain regions that would have served the missing parts are reassigned. There are also behavioural workarounds: Sanders Nellis, an “armless wonder” in P. T. Barnum’s 19th-century circus, could load and fire a pistol with his feet. Instead of pitying such folk, Blumberg argues, we should admire them, and accept that strict definitions of normality have no real basis in science.

“Strict definitions of normality have no real basis in science”

This is all good sense. What’s less clear in the book is what the abilities of individuals to adapt to anatomical quirks and environmental insults tells us about the evolutionary process that adapts animals to their environments. Some biologists think that our insights into development’s flexibility and robustness should cause a major rethink of how evolution works. Instead of seeing adaptation as arising solely by natural selection acting on tiny variations thrown up by random genetic mutations, it might be that development can generate variability suited to the environment, and that in some cases genes follow this adaptation rather than lead.

These views are controversial. Experiments in the lab show that evolution can work this way, but how often the same happens in the wild is unclear. Blumberg seems sympathetic to a development-driven view of evolution, but his exact position is vague, and this is the book’s weakness. Without a central thesis or strong narrative thread, Freaks of Nature reads rather like Blumberg’s description of embryonic development: “There is no single narrator and no internal dialogue. There is what just happened, what is happening now, and what comes next.”

This can make for a confusing read, but it is a reflection of real scientific uncertainty. Timely and wide-ranging, Freaks of Nature shows that although we’ve passed some exciting landmarks on our journey, we’re still some distance from that circled destination, and the route is still unclear.