The Clock Is Ticking: Can the Human Species Beat the Odds?
Have you ever wondered just how long the typical species lasts? After all, it is the fate of every species to one day become extinct.
But could technology possibly save humans from this fate? Or are we also dommed to one day disappear?
Let's take a gander.
Before we start, please note that evolutionary biology and palaeobiology are large and complex fields and that theories on extinction are ever-evolving (excuse the pun). The following article is only intended as a rough and ready overview, not a scientific thesis.
What makes a species a species and how do new species arise?
Let's start with a definition.
According to the Merriam-Webster Dictionary, a species is defined as: -
"A category of biological classification ranking immediately below the genus or subgenus, comprising related organisms or populations potentially capable of interbreeding, and being designated by a binomial that consists of the name of a genus followed by a Latin or Latinized uncapitalized noun or adjective agreeing grammatically with the genus name."
Simple enough, but at one point can we say that a new species has emerged? At what point can we definitively state, "Yes, now we have a new species"?
New species arise through a process called, appropriately enough, speciation.
This is traditionally defined as:
"Speciation is how a new kind of plant or animal species is created. Speciation occurs when a group within a species separates from other members of its species and develops its own unique characteristics." - National Geographic.
Speciation results in reproductive isolation from the original "parent species" population due to the accumulation of genetic differences between the two, or more, groups.
Speciation can occur in several ways. Perhaps the most common is through separation. This may occur when a part of a population becomes separated from the rest of their species, for example, through geological changes. Speciation can also occur without geological effects. In sympatric speciation, two or more descendant species rise from a single ancestral species. This may occur when different populations of a species begin to occupy different niches in the same geographic location.
Speciation may also be induced artificially, through animal husbandry, agriculture, or laboratory experiments. Genetic drift is another possible, and oft-debated mechanism for speciation. This is when genetic mutations introduce changes to a species over time.
Remember that a group of organisms is only ever considered a species if they can breed and produce viable, fertile offspring.
Of course, speciation doesn't happen overnight. Human action aside, it can take a great deal of time to accumulate enough genetic differences for a new species to emerge. In fact, it may be more appropriate to think of speciation as a continuum, and not a discrete, clear cut event.
The fossil record can also be quite deceptive when it comes to giving us insight into speciation.
Because the fossil record is incomplete, each new species can appear to come out of nowhere. This can be frustrating for paleontologists.
There is also another theory called "punctuated equilibria". Devised by the late Stephen Jay Gould and his colleague Niles Eldredge, this theory proposes that the evolution of new species can also occur in relatively rapid jumps -- often in response to major cataclysmic events.
This theory is not widely accepted, with prominent figures like Richard Dawkins, a notable critic, but it may occur in some instances.
Now, let's turn our attention to the modern world.
Given the slow rate of change needed for speciation to occur (except for artificial speciation, as when humans deliberately breed new species), it is difficult to "see it in action" today.
One example may be the Hawthorn fly, which some researchers believe may be undergoing speciation. A distinct population appears to have emerged in North America after apples were introduced in the 19th century. This new population feeds only on apples, and not on the fruit of hawthorns. Researchers have found some evidence that the apple-eating and hawthorn-eating flies now have significant genetic differences and are no longer inter-breeding.
The study of organisms with very short lifespans, like bacteria and viruses, can also sometimes illustrate speciation in action.
And that brings us to a nice segue, namely, what is the natural lifespan of a species?
What is the oldest known species, and what is the longest-living animal?
Let's address the first question, well, first.
You may be tempted to think of any number of so-called "living fossils" when addressing this question. But it is important to bear in mind that the term "living fossil" is a little misleading, and may even be considered a misnomer.
By way of example, let's take one of the most famous "living fossils", the coelacanth. Living examples include Latimeria chalumnae and Latimeria menadoensis, which are thought to be the last surviving species of the ancient clade Sarcopterygii (lobe-finned fish and tetrapods).
Here, the term "living fossil" is used as shorthand to describe the remaining extant species belonging to a distant ancestral lineage. If we were to analyze the genome of a modern coelacanth and compare it to that of its ancestors, stark differences would appear.
In fact, the term "living fossil" is more one of convenience than accuracy, and in many cases, it is an artefact of previous haphazard taxonomic classifications, and history of discovery, more than a useful concept.
But we digress.
Some of the oldest living species include species of the horseshoe crabs, which have been around for about 150 million years; species of horsetail plants, which may have evolved around 360 million years ago; and cow sharks, which emerged around 175 million years ago.
But what about the longest living individual animals within a species. Here are some common examples: -
- The average lifespan of a cat is between 10 and 20 years.
- The average human lifespan is roughly 80 years, but this can vary widely, depending on conditions.
- The average horse lifespan is anywhere between 25 and 30 years.
- The average elephant lifespan is roughly similar to humans.
- A lion's lifespan tends to be around between 10 and 15 years (in the wild).
- An average dog lifespan depends on size and breed, but smaller dogs tend to live longer at about 15-16 years.
- The average cow lifespan is about 20 years. But this depends on the breed and treatment in captivity.
All well and good. But what is the longest-living animal known today? The answer is hotly debated, but a few of the candidates are:
- Ocean quahog clam. The oldest recorded specimen was about 500 years old.
- Greenland shark. The oldest recorded specimen was estimated at about 390 years old.
- Bowhead whale. The oldest recorded specimen was about 211 years old.
- Rougheye rockfish. The oldest recorded specimen was about 205 years old.
- Red sea urchin. The oldest recorded specimen was about 200 years old.
- Galapagos tortoise. The oldest recorded specimen was about 177 years old.
Of course, some trees and other plant species are much, much older.
How long, on average, does a species last?
While many people frequently refer to our planet as "Mother Earth", "The Green Planet", or another nickname that conjures up fertility, sanctity, and vitality; the history of life on this planet is actually one of suffering and extinction.
From mass bio-extinction events to the daily loss of individual organisms within a population, death hangs above us all.
But, what are the typical "innings" of any particular species here on our home planet?
Setting aside cataclysmic events, like asteroid impacts, ice ages, eruption of super-volcanoes, etc., and human influence, there does appear to be an average species lifespan over geological time. This is referred to as the "background extinction rate" or "normal extinction rate".
It refers to the number of species that would be expected to go extinct over a given period of time, not taking into account human influence. The background extinction rate is generally measured over a particular period of time. It is sometimes given using the unit “millions of species years (MSY)” which refers to the number of extinctions expected per 10,000 species per 100 years.
The background extinction rate is different for different types of organisms. For example, mammals have an average species lifespan of 1 million years (although some mammal species have existed for more than 10 million). There are around 5,000 mammalian species currently in existence. This means that we can expect one mammal species to go extinct, on average, every 200 years or so.
Most of these calculations are based on the fossils record, which we have already seen in incomplete. In fact, the remains of terrestrial organisms are notoriously less likely to survive the rigors of time due to scavenging, weathering, and other environmental factors.
For this reason, the rate is more of a ballpark figure.
How often do species go extinct naturally?
As we have already seen, natural extinction rates are a little tricky to calculate.
Mammals, in general, of which we are a member, tend to have an average species lifespan on the lower end of the scale. Some types of organisms, however, appear to be far more robust. For example, marine invertebrates are thought to have a background level of extinction of 5 to 10 million years, while dinoflagellates may have a background level of extinction of around 13 million years.
These values, however, need to be taken with a pinch of salt. They should not be seen as concrete estimates, and mileage of some species within groups (like mammals) may vary widely.
It is also important to remember that extinction is a natural part of life on Earth.
After all, if the dinosaurs had never been toppled from their pedestal, our early mammalian ancestors would never have been given the chance to thrive and evolve into our very own species.
These historical estimates may also be widely off, as many soft-bodied organisms, especially microscopic ones, are very difficult to identify and track throughout geological history.
It also comes down to how niche-specific species are/were, how adaptable the species is/was, and in which habitats they lived (land or sea, for example), to name but a few issues. With all the problems associated with analyzing taphonomy (the process of fossilization), are there any estimates based on the living species today?
The answer is, of course, yes. But, you will also find widely different estimates for extinction rates here, too.
They range from something like 8,700 species a year (24 a day) by the Mıllenıum Ecosystem Assessment to somewhere in the region of 150 a day by the U.N. Convention on Biological Diversity.
These estimates are so much higher than historic rates because the extinction rate appears to be increasing rapidly due to human influence on the environment. Remember, scientists estimate extinction rates irrespective of human factors. This is because it is believed by most that humans have had a huge impact on the number of species becoming extinct.
For comparison, we only "really" know of roughly 800 extinctions which have occurred over the last 400 years. Of which, about 89 were mammals.
To put that into perspective, this is less than 1 tenth of a percent of the total estimates of global species count of 1.9 million. But, again, these are all guesstimates.
However, we do have another weapon in our arsenal that can help us here for estimating extinction rates ... genetics!
How long can humans expect to survive as a species?
There is a third way that scientists can estimate the lifespan of a species. By comparing the genome of distinct, yet closely related, species, it is possible to estimate, roughly, when they separated from a common ancestor.
In fact, this methodology has helped rewrite some of the long-held beliefs about the evolution of many species.
Using this technique, it has been theorized that humans (specifically the homo genus) split from the line that contains bonobos and chimpanzees between six and eight million years ago. Interestingly, bonobos and chimpanzees themselves split from one another around 1 million years ago.
What this means is that the "parent" species of both may have become extinct around that time, which fits the lower estimates for mammals from the fossil record.
Our species, Homo sapiens and our sister taxa, the now-extinct Homo neanderthalensis, split around 800,000 years ago, though new evidence suggests Homo sapiens and Neanderthal may have actually interbred, and thus may not have been separate species, but sub-species. We have yet to find any remains of our common ancestor, though many believe it may have been Homo heidelbergensis.
But, a recent study may be getting closer to finding out.
There is some evidence to suggest that the proportion of Neandertal-inherited genetic material is around 1.5 to 2.1 % in non-African populations, though some studies indicate it could be as much as 20%.
But this hypothesis is far from widely accepted by the scientific community.
If any of the above-mentioned estimates hold any water, that would mean we have somewhere in the region of at least 200,000 years of natural species lifespan left. But, this, of course, ignores completely our ability to adapt and use our technology to extend our time on the planet (or off it).
Homo sapiens have proved to be, so far, a highly adaptable species. According to many scientists, like paleobiologist David Jablonski, the human species is more likely than not to exist for a lot longer than would be otherwise expected, even if there is another mass extinction event.
In fact, we have been there before. There is evidence to show that Homo sapiens were almost wiped out around 70,000 years ago. The culprit? The eruption of the supervolcano, Toba in Sumatra, Indonesia.
This event is thought to have changed the climate to such a degree that the total human population may have fallen to between 5,000 and 10,000 individuals. Others estimate it could have been as low as 40 breeding pairs (but this is heavily disputed).
But this is, literally, ancient history.
How can humans save ourselves from potential extinction in the future?
To stave off potential extinction, like from supervolcanic eruptions, some believe we will likely need to leave the planet.
“The future of humanity is fundamentally going to bifurcate along one of two directions: Either we’re going to become a multiplanet species and a spacefaring civilization, or we’re going be stuck on one planet until some eventual extinction event,” Elon Musk said during the National Geographic Channel’s MARS, a global event series that premieres worldwide on November 14, 2016.
“For me to be excited and inspired about the future, it’s got to be the first option. It’s got to be: We’re going to be a spacefaring civilization,” he added.
If we can find a way off this world and populate the universe, this may be a way of extending our species' lifespan indefinitely. With the work of companies like SpaceX, Blue Origin, or any other of the private space companies currently operating, this may happen sooner than we think.
But even this might not "save" our species. As we have seen with other species on the planet, it may be the case that human colonies will evolve on their own path and quickly become new species in their own right. But that is likely to take another few hundred thousand years or so.