The coco-de-mer palm is a species of nut-producing tree found on only two islands in the Seychelles. It produces the largest seeds of any plant. Standing beneath a coco-de-mer palm when its nut is ready to fall is foolish. Each can weigh up to 20 kilograms and measure a third of a metre across; a direct hit on the head could kill you. By way of contrast, you would likely not notice if the seed of a silver birch tree hit you on the head. Each seed is 1–2 mm across and weighs only a fraction of a gram. Unlike the coco de mere, the silver birch is a tree that has conquered more than two islands – it is found throughout much of Europe and Asia. Biologists have not paid the diminutive size of silver birch seeds much attention, but they have been intrigued by the large size of the seeds of the coco-de-mer palm for years, and we now know why they grow so large.
The earliest palms evolved 80 million years ago, about 14 million years before the extinction of the dinosaurs. Since the evolution of the first palms, the coco de mere species has evolved to thrive in habitats with poor-quality soil where minerals and nutrients are scarce. The species has evolved characteristics that make it a specialist at doing well in harsh environments where individuals of other species are usually doomed to an early death. The cost of being able to live in such poor environments is that the coco de mere is outcompeted by other plants in higher-nutrient soils. What characteristics has the coco de mere evolved to be a specialist in low-nutrient conditions?
Evolution has shaped the leaves of the coco de mere to act like gutters, channelling water to fall near the trunk of the tree. When rain falls, it washes off bird poo, pollen and other bits of detritus that the leaves have accumulated. The rain, now laced with valuable nutrients and minerals deposited on the leaves, is channelled to reach the soil near the tree’s trunk. The minerals and nutrients soak into the soil, to be absorbed by the tree’s roots and used to grow new branches, leaves, fruits or more roots. The plant has evolved uniquely shaped leaves to capture nutrients and minerals that are scarce in the soil in the environment where it lives.
The coco de mere is also remarkably efficient at extracting nutrients from its old leaves before they are shed. No other plant species has been studied that is so good at reusing nutrients. The environment where the coco de mere lives is so harsh that all resources must be recycled if they possibly can be. Nothing should be wasted. But that does not explain why the seeds are so large.
The palm invests these valuable resources into huge nuts that take up to four years to develop, and two years to germinate once they have fallen. The coco de mere has evolved large nuts that are packed with nutrients to give germinating seedlings a great start in life. Seedlings of the coco de mere are so well provisioned by their seeds that they can quickly grow to a size where they can capture rare minerals and nutrients via their own gutter-like leaves. They start life with a built-in advantage over species with smaller seeds that cannot grow so large with the nutrients their parent tree has provisioned them with.
Large seeds do not fall far from the parent tree, and that is the case of the coco de mere, with most seeds germinating in the shade that their parent casts. What is truly remarkable is that coco de mere are the only plants known to nurture their young. Many animal species do this, but only one plant species appears to. Experiments have revealed that seeds which fall underneath their parent tree grow faster than those that grow further away. Part of the reason for this is that the guttering effects of the parent tree’s leaves channelling water and detritus close to its trunk also provide benefits to its nearby offspring. But that is not the whole story. Many trees actively produce chemicals to hinder seeds of their own species germinating under their canopy to reduce competition. Coco de mere do not do this, but rather parent trees provide an environment that actively helps their young.
Instead of only producing one or a few seeds each year, like the coco de mere, silver birch trees can produce hundreds of thousands of seeds annually. These tiny seeds have little wings, and they can be carried by the wind for many kilometres. Silver birch seeds rarely fare well under the forest canopy; instead they thrive when they end up in an open environment with little competition. This means that very few of them arrive in suitable conditions for germination. The lifestyle of the silver birch is consequently very different from that of the coco de mere. One produces millions of seeds, of which nearly all are expected to meet an early death, while the other produces very few seeds over the lifespan, but each seed has a reasonable chance of making it to become an adult. Plant biologists have used the scientific method – observation, hypothesis, experiment and repeat – to gain an impressive understanding of why particular species are the way they are, and why they have particular adaptations to thrive in the environment in which they are found.
You might never have thought about why a particular tree is where it is, but if someone had asked you to explain it, you would likely come up with hypotheses as to how its seed got there and why it germinated and developed into a sapling and then an adult. You could do this because at heart we are all scientists. We all construct hypotheses to explain aspects of the world around us. These hypotheses might include something as trivial as why someone blanked you in the street this morning, through to something as monumental as what is the meaning of life. Often you will have competing hypotheses to explain a particular observation. You don’t know why the dog vomited on the carpet, but it might be because he ate a discarded kebab in the park on his walk yesterday, or because he had a rabies jab two days ago.
Posing hypotheses is the second step in the scientific method after having made observations. The next step is collecting information to test the hypothesis. How is this done? What is good evidence, and what is not? How much evidence is enough? The scientific method involves more than observations, hypotheses and experiments. It involves knowing that your observations or experimental results are real and not just due to a chance result. How do scientists weigh the evidence and decide whether a hypothesis is supported or not?
Bold claims need strong support – if you claim to have a cure for cancer you need to be able to demonstrate it in a way that convinces other scientists. Part of the scientific method is being able to provide evidence to support your conclusions, and for other people to be able to repeat your work and get the same results. It is all very well to be able to come up with a hypothesis, report some observations or design an experiment to test it, only to discover that the data you collected are insufficient to test your hypothesis in a convincing manner. Let’s consider three examples of human-like species as examples of different levels of evidence.
Leprechauns are supernatural solitary fairies from Irish folklore. They are short in stature and – the males at least – are often depicted as being bearded and dressed in green. I might tell you, even in a way that sounds convincing, that a leprechaun inhabits my garden, but until I can provide evidence of his existence, you should not believe me. Just saying stuff isn’t good evidence. What evidence would you want? Ideally you would like to meet the leprechaun yourself or see his body, if he were to pass away. You might accept high-quality, undoctored photos or a genetic sample of a new species related to humans, but you should not take my word for it, however convincing I might sound. The great strength of science is it is evidence-based, and evidence needs to be produced in support or refutation of hypotheses. If hypotheses are supported in very large numbers of experiments, these hypotheses become facts. For the record, I have no evidence of a leprechaun living in my back garden. But that is no surprise, as how could he survive being trampled by my herd of unicorns?