This is the first of many blogs I will devote to considering some of the scientific evidence for where humans belong in the tree of life and what makes us unique.
They will set out to answer some fundamental questions that drive me in my science but also which are so basic that they touch each and every one of us in the profoundest of ways.
- Just what kind of an animal are we?
- What’s our place in nature?
- Who are our closest living relatives?
- Why are we so different; or are we?
- Was our evolution accidental (contingent) or driven by some deeper trend, process or influence?
These dominate my thinking and are central to my scientific research. The answers are deeply profound, excite me enormously, and I plan to share them with you in the blogs I write.
Hidden messages in origins stories
Each and every culture has had its stories about where humans came from, and they nearly always involved the first people being made by the hands of a divine creator.
Sometimes we were thought to have been formed from a substance such as clay or mud; or to have budded from a tree of life residing in the underworld; or even falling from the sky or being placed on Earth by a supernatural entity, already formed.
Whether we accept such stories as literally true or otherwise is not really the point.
They were stories told by our ancestors, and are sometimes still told today, because of the deeper meaning they hold for us.
They remind us of our profound connections with each other as human beings and also to the natural world.
Modern science offers us a purely physical account of human origins; that is, one without the presence or action of a supernatural force.
Yet, it also contains a profound message about connections and shared history; and one that runs much deeper, and is more moving, than any myth ever told.
It is an account founded in real physical evidence accumulated by biologists, anatomists, archaeologists and geneticists, and verifiable by anyone with a little training; and without having to accept wisdom handed down through the generations from some higher authority, human or otherwise.
We now have sufficient account from the material processes we see around us and experience in nature each and every day to explain how we humans began.
Scientists don’t resort to explanations involving supernatural entities or forces because they would require us to make untestable assumptions, and also because they raise many complex and unanswerable questions.
- Where did this presumed supernatural force come from?
- Where does it reside?
- How does it work?
- If it is conscious, why did it create us?
- What are its motivations?
I’ll leave these to the theologians.
The physical evidence we have for our evolution is complicated, to be sure, but it is amendable to examination and testing through common sense and through the laws, theories and techniques scientists have established since the Renaissance.
The scientific method has shown how humans are made of the same basic stuff – chemical molecules, structures, organs and bodily arrangements – as other life: information missing from traditional accounts of creation; unless we take “mud” or other such substances as a metaphor, which we might reasonably do.
It shows how humans evolved in accordance with universal mechanisms – including natural selection and random genetic drift – that have affected all species alive today and in the past.
Moreover, it shows in the deepest way possible, right down to the basic molecules that grow, shape and sustain our bodies, how we humans share a common history with all other life on planet Earth extending back some 4.5 billion years.
There is no deeper connection than what we see in the chemicals we carry in each and every cell in our bodies, the blood pumping through our veins and the organs that allow us to move, breath, eat and think.
A very primate heritage
So, just where in the tree of life do we belong then?
We humans are a primate: the “leading” animal, or group of mammals, 18th Century naturalists and philosophers thought was closest in resemblance to God (in the church, the “primate” is the head priest).
The Order Primates, as used as a category in science today, includes all living and extinct lemurs, lorises, tarsiers, monkeys and apes.
Yet, even as far back as Ancient Greece the philosopher Aristotle (384-322 BC), and in Ancient Rome, Pliny the Elder (c23-79 AD), described strong resemblances between humans and monkeys.
It is to these Greco-Roman philosophers that we owe an even more fundamental debt as scientists, for they first proposed the proposition that nature is governed by natural laws that can be investigated and understood through an inquiring mind and human reason.
Just what are these similarities between us and the other primates found hundreds of years ago? Like other primates, we have (for example):
- Grasping hands and feet, with five digits on each hand and foot, and a grasping big toe or thumb,
- Our skeletons have two forearm (radius and ulna) and two leg (tibia and fibula) bones,
- We can rotate our forearm bones across each other – think of the movement when you have your palm facing upwards and then turn it to face downwards, this is common to primates,
- Our skulls comprise very similar bones, with our eye sockets enclosed by a bony bar,
- The muscles that move our skeleton are very similar in number, arrangement and function,
- We have fingerprints,
- Our brains are large,
- Our eyes face forward, and we have a well developed sense of vision, but relatively poor sense of smell,
- We share a defined period of adolescence with a growth spurt, and
- We have complex social lives.
Just aping about
Without doubt, humans are an ape: a fact established way back in the 18th and 19th Centuries through the dissection of monkey, ape and human cadavers, demonstrating profound physical similarities between us all.
The French naturalist Comte de Buffon (1707-1788), for example, observed:
“At what distance from man shall we place the great apes, which resemble man so perfectly in bodily conformation?”
Comparisons of the DNA contained in the nucleus of our cells, the very molecule of inheritance, shows the human genome to be most similar to the DNA of living chimpanzees, and in turn gorillas, then orangutans, then the gibbons or lesser apes, then African/Asian monkeys and so on.
We humans share with the apes the following features (note that some of them are shared also with monkeys; list not exhaustive):
- Colour vision,
- Stereoscopic vision (or seeing in 3-dimensions),
- Eye sockets that are completely enclosed by bone, except where they open for us to see out of course,
- No tail (or lack of caudal vertebrae),
- Shoulder and hip joints that are capable of very wide ranges of movement,
- Our shoulder joints are placed to the sides of our bodies, rather than beneath them,
- We have 32 adult teeth,
- Teeth with relatively simple surfaces, essentially adapted for fruit eating,
- Broad faces with wide noses and jaws,
- Large brains compared to the size of our bodies,
- An intimate association between the developing fetus and the maternal blood stream,
- We most commonly give birth to single offspring,
- Large body sizes: especially so in the great apes (orangutans, gorillas, chimpanzees and humans), typically from around 30 kg to 200 kg depending on the species and sex,
- Female monthly reproductive cycle,
- Long life spans, with each segment (gestation, infancy, adolescence, adulthood) comparatively long, and
- Very complex social lives.
Friderun Ankel-Simons in her book Primate Anatomy has a fascinating and revealing illustration of the amount of time each stage of life occupies as a percentage (%) of total life span in various primates:
It tells a powerful story in its own right, highlighting similarities between us and other great apes, as well as helping to explain the differences among different primates.
We humans are clearly exceptionally similar, as a species, to chimpanzees in terms of our lifespan and life pattern, with about 2-3% of our lives spent in the womb, 8-9% as infants, 18% as juveniles and 70-72% as adults (Note: the human data would be from traditional, hunter-gatherers).
Thus, the time we spend learning as infants and juveniles is at the expense of our adulthood (i.e. we loose time because of it), but it is absolutely essential, as apes, to our learning the necessary skills we need to succeed as adults in a complex world.
A matter of complexity?
With so many similarities between us and other apes we are faced with a conundrum.
How is it that humans can be so different, especially in terms of our behaviour, when we are fundamentally (biologically) so similar to other apes?
This is a vast topic, spanning many areas of science and even philosophy, and one I can’t possibly do justice here.
Instead, I will tease it out across many blogs, getting at it from different angles and points of view, from the perspective of an anthropologist studying human evolution.
To make a start though, one old idea, going back to at least the 19th Century, is that evolution is characterised by “progress”; with organisms progressing from simple to complex through time, with humans epitomizing this process.
Even 20th Century biologists held the view that complexity resulted from the accumulation of DNA changes driven by natural selection over billions of years and culminating in human intellectual and technological sophistication.
It’s a view that holds sway even today in some quarters of biology, and permeates anthropology.
But, for those scientists unfamiliar with the amazing new information emerging from the burgeoning field of genetics, especially in the genomic era, it will come as a shock to learn that this view has been shown to be fundamentally wrong.
One way to think about complexity is to simply count the number of genes in the genome of an organism. According to this old idea of progress, used by 20th Century biologists, more complex organisms should have vast numbers of genes, well in excess or simple organisms.
Until recently, geneticists were unable to estimate with any accuracy the numbers of genes present in the genome of most organisms; it was only with the availability of gene sequencing from the 1990s onwards and the dramatic increase in computer power and drop in cost that this has become possible.
So, let’s take a look at some examples:
|Organism||Number of genes (approximate)|
|Malaria parasite and yeast||5,000|
|Opossum||18,000-20,000 (estimated range)|
|Monkeys and domestic horses||20,000|
|Chicken||20,000-23,000 (estimated range)|
|Humans, chimpanzees and gorillas||21,000|
|Rats and cattle||22,000|
|Arabidopsis (flowering plant)||25,000|
|Maize and grape vine||30,000|
|Tomatoes and potatoes||35,000|
It’s pretty clear from the list above that humans are nothing special in the gene complexity stakes.
On the contrary, rats and cattle, flowering plants, food crops like papaya, maize, grapes, tomatoes, potatoes and rice, and even the aphids that feed on economically important plants, have more genes than we do!
The study of DNA, especially those genes associated with organs such as our brain, does offer much promise in the search to understand why humans are so unusual.
But this research is beginning to show that even the same genes can function quite differently in the brains (and other organs) of different species; so, even having the same genes doesn’t mean your body or behavior will be the same.
There is a real caution here also for those scientists studying the DNA of our extinct relatives like the Neanderthals, for even if they have some of the same genes as us, they might have done quite different things physiologically on account of them sitting within a different genome.
But, simple exercises like tallying the number genes obviously offers no real insights into such important questions; despite what many biologist thought for many, many, decades.
Moreover, the old idea of evolutionary progress from simple to complex cannot explain the evolution of life in general, let alone humans, and fails to account for our uniqueness.
Promising paths for research
Ultimately, the search to understand why humans are so different to other organisms – with our complex and symbolic language, sophisticated culture, and heavy reliance on technology – can only be understood through varying and complimentary perspectives.
Some of this understanding comes from looking at our shared heritage with other primates: we would be nothing without our grasping hands, large brains, stereoscopic and colour vision, small and simple teeth, capacity for language and culture, use of tools, and rich and complex social lives.
Yet, these primate features one their own do not explain why humans are so unusual; why we, and only we, create art and literature, build cities, invent religions, practice science, and explore the solar system and beyond looking for clues about our identity and existence.
First published in April 2014.
Posted in: Incredible human journey