The tree of life starts from LUCA and describes how new species evolved from existing ones. It is not complete, and there are many gaps, particularly the further back in time we go, because species alive in the distant past are extinct, and for many of them no fossils have been found from which they can be described. Nonetheless, it is clear from even the incomplete tree of life that different now-extinct species had very different fates. Some produced many descendant species that are still alive, while others left none. Anchiornis huxleyi was one of the first species of birdlike dinosaurs, evolving about 160 million years ago, and even though it is long extinct all of the ten thousand or more species of bird alive today are thought to be descended from it. Tyrannosaurus Rex is at the other extreme, leaving no descendant species, as its lineage went extinct following the meteor impact that brought the Cretaceous geological epoch to an end 66 million years ago.
We share nearly all of our line of descent from LUCA, and most of our DNA sequences, with chimpanzees and bonobos. The common ancestor of our species and of these two great apes walked the Earth up until 6 to 10 million years ago. If we accept an estimate of 3.9 billion years since LUCA lived, then humans, chimps and bonobos had identical lines of descent for 99.8 per cent of the time that life has been using the genetic code. For 90 per cent of those 3.9 billion years, humans and fish had identical lines of descent, and for approximately 75 per cent of them we shared a direct line of descent with bananas, rice and tulips. For between 15 per cent and 20 per cent of those 3.9 billion years we had the same line of descent as bacteria like E. coli. We share progressively less of our genetic sequences the further back in time we have to go to find a common ancestor with another species. Our DNA is consequently more similar to that of species of fish than it is to species of plants. What the tree of life reveals is that life has been diversifying and proliferating for a very long time. You are descended not only from apes but also from fish, jellyfish-like beasts, archaea like strain MK-D1 and bacteria.
The tree of life that biologists have drawn is remarkable. I can spend hours looking up species that I was unaware existed, and that in many cases no one has studied. As one moves forward in time from LUCA we first encounter bacteria, which then threw off a new branch, the archaea, which in turn split into two to produce the eukaryotes on a new branch. There are then endless splits that encompass yeast, plants and animals, until eventually we find Homo sapiens at the end of one of the branches. Sitting next to us on neighbouring ends of branches are chimps and bonobos.
It seems unlikely biologists will discover a new domain of life, so I expect these three major groupings to remain, but scientists are continually discovering new extinct and extant species in each of the three domains and will continue to do so as we catalogue more of life’s remarkable diversity. Despite being unfinished, at the time of writing, the tree of life contained over 2.2 million species with nearly all of these organisms consisting of only a single cell. We are unsure quite how many species there are still to be discovered, but as we find them, they will fit somewhere into the tree of life.
Each species alive today differs from others not only in their genetic sequences but also in what they can do. Strain MK-D1 is able to live off the waste of hydrogen-gobbling bacteria, but I cannot. In contrast, I can just about run a mile, while strain MK-D1 cannot. I have a very different body shape to Woofler, or the bird singing in the silver birch tree outside, and these body shapes map to things we can do. I can hold conversations, climb a tree and peel a banana. Despite my attempts to stop him, Woofler can effectively chase down pigeons and squirrels and expertly dispatch them in a way I cannot. Neither Woofler nor I can suck nectar from flowers and we would be unable to repeatedly migrate 3,000 miles from Alaska to Mexico and back each year, a feat the 3.5-gram rufous hummingbird achieves in each of its three to five years of life. Each species is different, and each is adapted to a particular way of life.
Woofler can effectively hunt small prey, I can peel a banana and hold a conversation, and a rufous hummingbird can fly the length of the North American continent because we have attributes that enable us to do these things. I have dextrous hands, a voice box and a large brain; Woofler has sharp teeth, keen eyesight and a very strong hunting instinct; while rufous hummingbirds have long bills that allow them to feed on high-energy nectar deep inside flowers and wings that let them hover and migrate. Each species has characteristics that allow it to do a specific set of tasks that enable it to find food, survive and reproduce. Biologists call these characteristics phenotypic traits, and they are defined as the attributes of an organism you can measure. Phenotypic traits range from specific molecules that individuals produce through to an individual’s body weight or aspects of personality.
Phenotypic traits differ between species. Dogs look different to people because they have a different set of phenotypic traits. Different individuals within a species can have different values of the same phenotypic trait. For example, you may be taller than me, or shyer, or have different-coloured eyes. Differences between species in the phenotypic traits expressed, and differences between individuals within a species, are the result of genetic differences. If we have different blood types from one another it is because we have different alleles at one or more of our genes. When individuals with different phenotypic trait values have different chances of surviving and reproducing because they differ in their ability to detect, acquire and utilise resources such as food, natural or sexual selection occurs, and this can result in different generations having different frequencies of specific alleles. Such genetic change across generations is evolution.
An individual’s DNA sequence contains the code to produce proteins that determine how an organism develops, and the phenotypic traits it expresses. Phenotypic traits are important because they determine what an individual of a particular species can do, while the DNA sequence determines which phenotypic traits an individual of particular species will develop. The study of phenotypic traits has been the focus of much of my career post-Ph.D. Given my graduate studies at Imperial College London had focused on how animals impact the distribution of trees by dispersing and consuming seeds and seedlings, it might seem surprising that I switched tack and started studying phenotypic traits and evolution as I got older. I am glad I did, but the route I took was not carefully planned.
Woofler’s Taxonomy
As I neared the end of my Ph.D., the realization that I would need to get a job dawned, and so I started to think about next steps. I knew I loved science and wanted to understand why I existed, but I didn’t know the best way to achieve this. Was it to become an academic, working on one small part of the problem while reading up about other aspects, or should I take another job that might give me flexibility to do the reading I needed to do? I didn’t really know. The first job I applied for was a postdoctoral researcher position at the Institute of Zoology, the research arm of London Zoo, where a postdoc is the next stage in academia after completing a Ph.D. Postdocs are typically employed on contracts that last for between one and five years with the job description outlining a particular scientific challenge that should be addressed. They are usually advertised by a university faculty member who has succeeded in securing funding from a government grant or a charity following a highly competitive peer-review process. As well as this job, I also sent off for forms to become a weatherman at the BBC, and for the civil service fast track, where postgraduates are trained to become scientific advisers within government departments. The detailed description for the weatherman job revealed I would need to be more interested in the weather than I was, and the civil service fast-track application procedure was so long and drawn out I could never motivate myself to start filling out the form, so the only job I ended up applying for was the postdoc position at the Institute of Zoology. I was amazed when I was offered it, as the interview was disastrous.