Why Are We The Only Human Species?

Polar Bears, Panda Bears, Grizzly Bears, Brown
Bears, Paddington Bears… So many species of bears. But bear with me with the puns. Why
are we the only human species? Many Animals usually don’ t come in just one
flavour, but Homo sapiens or ‘homo sapiens sapiens’ if you’re into the sexy trinomial
‘Genus species subspecies’ nomenclature, are rather alone. With all our ingenuity and opposable
thumbs shouldn’t humans be more diverse than our house cats? This is called the ‘plurium interrogationum’
fallacy. Quite the engaging fallacy, but not a great tool at answering a question. Instead, let’s go back in history. No. Pass
the renaissance, yes, way pass the Middle Ages, no I m talking way back. Uhh, yeah okay hold on…
no that’s too early, yes somewhere there! Yes. Let’s start here, the Palaeolithic period.
That’s generally a good place to start . Hominid species started making stone-tools around this time
(paleo- lithos). See? It’s in the name. Now sure, we like tools and this is significant
for our human development, but nothing much happens in these early days except for updating this
stone tech from mode 1 to mode 2 and starting to band together as tribes for the next couple
of million years. So let’s jump to the end of the lower Palaeolithic age. This is also
probably when early hominins mastered fire and started the transition to true civilisations.
But this period is more important looking at our question because this is when truly
anatomically humans start migrating out of the African Savannah. Leading up to this point our earlier ancestors
were mostly vegetarian. There is no evidence that they were systematically preying on large
animals before they moved out from the forests to the dry flat savannahs. This change in
the Palaeolithic period, as their stone tools would attest, proves that hominins were becoming
increasingly carnivorous. But as you might work out, more meat requires an even greater
amount of plants to sustain their prey. These times are marked by the species such as Homo
habilis who used simple stone tools for hunting small prey. So overall there’s less food to
go around and this increasing competition for a finite amount of meat could have driven
many competing hominin species to go extinct. But this surely does not explain why we have
only one species today. In fact, until quite recently, we shared the planet with other
human species. In the middle Palaeolithic period, things
started to slowly pick up. So as I mentioned, the anatomically modern humans we now identify
started to really show up during this period. Anatomically, modern humans can generally
be characterized by their lighter build of their skeletons compared to earlier humans.
But what is more significant is the radical enlargement of, our big squishy brains or
in this case, Homo erectus’s to an average size of approximately 1100 cubic centimetres
which was a great jump from Homo habilis’s 610 cubic centimetres. These times also started
showing up with hominids with early behavioural modernity in terms of what we recognise as
modern human social behaviours. These are traits such as figurative art, burial, fishing,
use of pigment and jewellery and the emergence of composite tools. The point why this difference between anatomically modern humans has relevance in the discussion is
features of anatomical modernity could be found in evidence spanning much longer than
that is available for behavioural modernity. And this is important to answering what we
consider to be human. Consider the australopithecine fossil ‘Lucy’ that was named after the famous
Beatles song. Lucy is dated to have lived somewhere around 3.2million years ago and
her pelvic and knee bones clearly prove that she walked erect like we do. But her brain
size was the size of a modern chimp around 500 cubic centimetres and didn’t exactly behave
like a modern human. And that’s why we have to wait until the end of the palaeolithic ages to see
the full brain development this bipedalism brought about in the hominins. Palaeolithic ages bring us up to two competing
but related theories to the origin of modern humans: the Homo sapiens. That’s right! two theories… you have
been deceived. Kind of. First is the more famous and favoured out of the two, which
came up in my oldest language video, (go check it out, I’m shameless) This is the Out of
Africa Model, or more accurately the recent African origin model; this model asserts that modern
humans evolved relatively recently in Africa, and then migrated into Eurasia and replaced
or assimilated all previous populations who had descended from Homo erectus. So let me
try to be very careful here, this theory proposes that after Homo erectus migrated out of Africa
less than 270000 years ago, the different populations became reproductively isolated,
evolving independently, and in some cases like the Neanderthals, into separate species… The competing Multiregional Continuity Model has a slightly
different proposition, that after Homo erectus left Africa and dispersed into other portions
of the Old World, regional populations slowly evolved into modern humans, this is a subtle
but an important difference as you see. This theory argues that all the diversified humans
we see today moved out of Africa much earlier than what the Out-of-Africa theory suggests,
probably some 2million years ago, and the regional variation we see today is due to
the natural evolutionary pressures of those geographical regions. Now, this multiregional theory has gained
more support since it was first proposed back in the 1980s and we do not know where the
consensus will end up as more evidence accumulates in the future. But one thing is for sure,
Homo sapiens were not alone. We’ll get back to the speciation in a bit, and look at what
we do know, around say 50000 years ago (yes let me be super vague about these timelines because
I do not want any angry Paleoanthropologists as my enemies): we know that during this time
at least 3 other hominin species were around: the Neanderthals in Europe and western Asia,
the Denisovans in Asia, and the so-called “hobbits” from the Indonesian island of Flores. Why 50000 years ago? Glad you asked. This
is because we see our own homo sapiens to develop behavioural modernity around that
time although anatomically being modern for a much longer time. Homo floresiensis or the hobbit bones were
discovered in 2003 This hominin is incredibly small
reaching a height of about 1.1 meters. And its brain size even smaller than that of Homo
habilis s we met earlier around 380 cubic centimetres. But size is not everything, because
a better indicator of intelligence is the Brodmann’s area 10, the dorsomedial prefrontal
cortex, an area of the brain associated with higher cognition. This is why even though
sperm whales have a brain 8times as large as ours, they keep failing to produce a chess
grandmaster. Homo floresiensis region 10 is about the same size as that of modern humans,
despite the much smaller overall size of the brain. And this is consistent with the observations
in their cave dwellings that suggest behavioural modernity with evidence of cooking, cooperative
hunting and butchery. We know even little about the Denisovans.
Their discovery was made back in 2010 and how that discovery was made was what makes
the Denisovans very unique. Unlike the traditional archeologic discoveries they were discovered
in a genetics lab he Max Planck Institute for Evolutionary Anthropology in Leipzig,
Germany. The scientists sequenced the mitochondrial DNA, using a finger bone fragment of a juvenile
female who lived about 41,000 years ago, found in the remote Denisova Cave in the Altai Mountains
in Siberia. The Paleoanthropologists are still reluctant to give a scientific name yet because
we need to identify more fossil samples of the Denisovans before we jump the gun. Some might want to also add Homo Naledi to
this bunch. But like the Denisovans, we know little about them. Their behavioural modernity
is also not very well understood. Because Naledi was discovered in 2015 in South Africa.
We are beginning to understand more about these hominins due to more fossil being samples
discovered since the initial discovery. But the most important reason we don’t include homo naledi in this list is because their samples are way older than
the 50000year mark we discussed. going back at least 235000Years At this point, however, we do know quite a
bit about our cousin Neanderthals, because we have known about their existence pretty
much since the 19th century. And they might give us the best clues as to why we are alone
today. The archaeological evidence strongly suggests that the Neanderthals somehow lost
out to modern humans and some newer evidence suggests that we also started interbreeding
with them, which to put in another way is to say that they never really went extinct… But if this was a race we can’t exactly say that Neanderthal genes won the race. uhhh…And that cousin
comparison is certainly not apt anymore. But this does solidify the multiregional theory
that the different human species did not live in isolation as the out of Africa theory suggests. Anyway, Neanderthals evolved long before us
and lived in Europe well before we arrived. By the time we got to Europe, just over 40,000
years ago, Neanderthals had successfully been living there for over 200,000 years, becoming
more adapted to the colder European climates than we clearly are. Neanderthals were
formidable hunters with sophisticated tools. But they too seem to have struggled
more with an enemy we have in common today… Climate change. As Europe experienced colder
climates they seemed to have vanquished along with it. But as I noted the cold itself was
not the problem. They were pretty adapted to it and wore warm clothes to fight the chill.
More likely it was the rapid changes in the landscape that they appear to have struggled
with. You see, the Neanderthal hunting strategies and style were specialised to hunt larger
game in woodland environments, unlike Homo sapiens who had a wider more generalised range
of hunting capabilities. As Europe started to look more like a cold African savannah,
Homo sapiens found right at home because humans were adept at hunting smaller mammals like
rabbits and hares and preferred the open conditions. But the natives found their forests which
had sustained them thus far to rescind and fade away. The Neanderthals struggled to innovate
new tools and techniques that could bring down the new abundant smaller species. Also, Jean-Jacques Hublin of the Max Planck
Institute for Evolutionary Anthropology points out to the evidence that although Neanderthals
were highly capable and efficient at their speciality, Homo sapiens might have had an
intrinsic quality to adapt quickly. But more importantly, we might have had an upper hand with climate change
due to a something very unsuspecting. By the year 40,000BCE, humans in Europe were making
things any of us would recognise as art. this is a very important
piece of art. It is the Löwenmensch Now if you thought
this was made during the dawn of our civilisation, well guess again. Thanks to carbon dating,
we know that this piece of art is around 350000years to 40000years old. And this is very significant
because although Neanderthals and other human species might have been highly organised and
evolved, they don’t seem to have organised their social units to convey abstract ideas
such as they did with this lion-man figure. Symbolism is a very advanced concept as far
as intelligence goes. As Homo sapien numbers swelled, we began living in much more complex
social units and needed more sophisticated ways to communicate, share information and
organise around identities. Symbols and Idols are powerful ways of such organisation. To
put it another way, art and symbols are a strong social glue that unified them and helped
organise their social and economic affairs with one another. And unsurprisingly, symbols
can help a lot with transferring ideas and knowledge from one generation to another.
The lion man sculpture is so complex for that time that researchers estimate some 400hours
put in by a specialist. And that points to the importance of that figure in the tribe
and level of commitment as a tribe to sustain the craftsman in a period of time where glaciers
were creeping in and food sources were dwindling. We can also find many other examples such
as this gryphon vulture bone flute from the same period. In contrast, Neanderthals seem to have had
limited need for such display. To be fair, there is evidence of Neanderthal jewellery
but not such an extensive organisation in symbolic information sharing. Neanderthals
had comparable brains sizes to modern humans so why we were so artful and they were not
is the next question we need to answer to solve this puzzle. One proposition is that for the 1st 100000years
since Homo erectus left Africa all the species acted in a similar way until some stressors
changed the homo sapiens’ genetic makeup. And there’s evidence to suggest that. There
are some dozen of changes in our genetic makeup that we do not find in the Neanderthal or
the Denisovans, several involving brain development. Humans have a curiously long brain development
stage which has a lot of interesting implications. We have come up with elaborate social structures
to care for our young who are very dependent on their parents for a long time. This extended
period of brain plasticity also gives our species an abnormally long period of learning
and a need for adapting to our environments which in turn results in a higher level of
cognitive abilities. It also results in a need for these hunter-gatherers societies
bring in way more calories that they would otherwise need, and additional need for improving
and learning. I urge you to read the paper by Professor Jean Jacques Hublin on the matter
because it is truly fascinating. Anyways… This ingenuity in the face of change is nothing
new. Homo erectus was massively successful until one or few stressors gave rise to the
modern humans. Homo erectus moved in small groups probably about 100 or so and reached
as far as the Indian subcontinent just in time to witness a truly cataclysmic event.
70000 years ago the Supervolcano Toba erupted in what is now Sumatra in Indonesia. The impact
of this eruption could be compared to a mile long meteor hitting our planet surface. The
Volcanic ash initiated a phenomenon called a ‘volcanic winter’ lowering the planet temperature
by about 15 C degrees and the peak of the devastation lasting for decades. This could
have wiped out most of the early humans or Homo erectus, according to what model of migration
you choose. But it is consistent with the genetic evidence which suggests that we descend
from a 1000 to 10000 endangered breeding pairs some 70000 years ago. This genetic bottleneck
is also observed in many other species such as cheetahs, gorillas and orang-utans. So
these remaining band of new mutants were truly the fittest in terms of evolution to adapt
and meet their very competitive lifestyles. And perhaps there lies the final piece to
this puzzle to explain why we are the lonely surviving species of our kind. More than half
of all living primate species are threatened with extinction. Out of which all the 6 great
apes are threatened and 4 are critically endangered. Hominids lives are complicated: they live
in complex social structures that are very vulnerable to changes, their brains require
a large amount of energy and decades of development, their babies are very vulnerable and the list
goes on. So if you think about it we are pretty hard to maintain as a species, and that is
not usually the best thing in a cut-throat world of evolution. We could think that we
are special that we turned out to be the exception among our relatives. Or look at it from the
perspective of millions of years of hominid evolution and look at our friends who perished
or assimilated and wonder if we are just another bubble in the cauldron of evolution or we
just got really really lucky? This is the beginning of a series and next
time we are going to continue this journey to put the development of human self in perspective. I’ll see you next time. Oh and I forgot Black Bears, Sun Bears, Sloth
Bears, Spectacled bears… You know, gotta represent!

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Reader Comments

  1. Kyle Duncan

    Great video, but please: There is no such word as "homo sapien" without an S at the end. The word is HOMO SAPIENS. The s at the end is not a plural s, just like the s at the end of for example homo erectus is there in both singular and plural. This may seem nitpicking, but when you're doing science stuff, you need to be careful, especially with such an important term, otherwise it is impossible to take the rest of the video seriously. Say after me… "I am a homo sapiens". 😉

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