In 1985 the 1,200 inhabitants of Potosí, a small town in the Venezuelan state of Táchira, were instructed to abandon their homes. Soon, water from the Uribante River would flood the silent streets, eventually brimming in an enormous natural basin defined by the surrounding Andean mountains. This was the new Uribante-Caparo reservoir, established to fuel a hydroelectric dam and provide power to the state. Meanwhile, Potosí became little more than a memory, marked only by a haunting crucifix penetrating the water’s surface. Former residents would have recognised this as the apex of an 82-foot-tall Spanish colonial church now submerged beneath. Although the history of Potosí had been swallowed whole, the façade of the sunken church began to re-emerge in 2008 in a devastating drought that continues to starve the reservoir, and the country, of water. Time has been rewound, and the dilapidated church now, once more, stands dry and exposed.

The story of Potosí is an incredible incidence of history being uncovered through physical deconstruction. Less dramatically, one could learn about the previous residents of an old house by peeling back layers of wallpaper. In both cases, the embedded history is dependent on modification over time, rather than starting afresh – the Spanish colonialists who erected the church of Potosí certainly didn’t do so as a novel means to dictate the volume of water that could later fill the reservoir. For them, the church was the finished product. When it came to filling the reservoir, it was probably more cost- and time-effective to leave the church in place than to destroy it. After all, the dam could function perfectly well either way. While this almost makes sense as a design principle, embedded history is highly unusual in manmade structures. For example, we would expect deconstruction of an internal combustion engine to tell us about how the constituent parts engage to propel a vehicle forwards, but not inform of the mechanisms that powered historical locomotives. Before the engine, horses were widely used to pull along carts and wagons. However, they certainly don’t feature anywhere in its construction (aside from the terms used to describe their power). The engine could now, in theory, be completely re-engineered from the bottom up such that the product hardly resembles its predecessors, or replaced altogether with a machine that better performs the same function. History inspires new designs, but does not constrain them. It is not inherent to the object, and so is lost, or at least relegated to museums and history books, as new and improved designs emerge.

As biological machines, and successful ones at that, we might assume that humans abide by similar rules as engines – over evolutionary time, our anatomical cogs and gears have been re-engineered and replaced in a process that identifies best solutions to environmental problems. But this is not the case. Although the analogy quickly breaks down, animals are probably much more like sunken churches. The 19th century biologist Ernst Haeckel was one of the first to consider the relatedness of human development and evolution. He argued that ‘ontogeny recapitulates phylogeny’ – an attractive idea that evolutionary transitions are played out in developmental events. Thus, on their way to acquiring human form, embryos would first resemble pre-historic filter-feeding plankton, fish, amphibians, rodents and early primates. The arising implication was a scala naturae in which humans were both maximally evolved and maximally developed, and other animals thus aborted forms along these scales. Haeckel’s controversial biogenetic law is now widely discredited. Instead, as proposed by the embryologist Karl Ernst von Baer, humans can resemble the embryos of their ancestors in their development because general characteristics of a phylum emerge before the species-specific elaborations set in – a basic five-digit pattern is formed at the end of the limb before it is sculpted into a human hand, a tiger’s paw or a bat’s wing. In each case, a common primordial structure, in this example the digit plate, is differentially modified to generate a species-specific anatomical component.

Evolutionary transitions are played out through modification of the parts available in the embryo, and so it is not surprising that, in some cases, they are replayed during development. A remarkable example is the transition to terrestrial life. In fish, the gills are derived from a series of swelling on either side of the head called the pharyngeal arches, and, in many cases, they are covered by a posterior projection of the head. This structure, called the operculum, ensures effective gas exchange by generating a one-way-valve for the movement of water. It is formed through overgrowth of the second pharyngeal arch. In amniotes, that live on land, there is no operculum. However, it is still found in the embryo. The second pharyngeal arch expands as it does in fish, but it goes further and eventually fuses to define the external contour of the neck. The embryonic operculum has been redeployed to seal the neck, enclose its glandular contents, and mark a commitment to terrestrial life. Evolution has not generated anything new. Rather, it has made best use of the available parts in the embryo. Our fishy past is hidden from sight, but inextricably locked in our development. The church is submerged beneath the water’s surface. Interestingly, pathological failure of the arch to fuse can generate openings in the neck, called branchial fistulae. The reservoir is drained and the spire unearthed.

The opercular flap of a fish (left) and the external contour of the adult human neck (right), both formed through expansion of the second pharyngeal arch during embryonic development.

Our evolutionary history defines and constrains us. Although the history of Potosí physically manifests in the Uribante-Caparo reservoir, the analogy collapses because the church is disposable. It could have been demolished before the town was flooded with no consequence for the reservoir aside from its ability to hold marginally more water. The history, however, persists despite gross changes in the structure and function of the valley. The difference with animals is it is impossible, in evolution, to wipe the slate clean and start again. It is not, and cannot be, a matter of design. Novel anatomies can only emerge through descent with modification – the imposition of small tweaks in the developmental programme that alter the shape and size of anatomical components in the finished article – and so the preservation of history is not an eccentricity, it is a necessary by-product of natural selection acting on the parts available. With this, evolution and development are coupled. History dictates what is possible in the present, and is embedded within it.