Post on 27-Jun-2020
LLL Innis 2018
TERTIARY AND QUATERNARY CLIMATE CHANGE AND THE
EVOLUTION AND SPREAD OF HUMANS.
Today’s uplifting message!
Last week we saw that a record of
climate change extends back about 4
billion years. It’s a record of
changing sedimentary
environments and the emergence
and evolution of life. The record is
limited until the last 600 million
years; since the Cambrian and the
apparently rapid diversification of life
known as the Cambrian Explosion.
The record of biodiversity through the last 600 million years indicates a logarithmic increase in species through time. However, the increase is not a smooth one. It is interrupted by several mass extinction events at which biodiversity is drastically reduced – the Big Five mass extinctions. The largest occurred at the end of the Permian when perhaps 90% of species went extinction. The latest one, at the Cretaceous-Tertiary (K-T) boundary, had about a 70% loss.
All except the K-T extinction appear to
have been caused by a complicated set of
mechanisms determined by plate
tectonics. At the K-T boundary we can
make a good case for asteroid impact as
the cause.
That boundary is very significant for us.
From that time we see modernization of
the biosphere – the rise of mammals,
insects, birds, flowering plants, etc.
Evolution of animals V= birds, BB= placental
mammals
Evolution of plants and animals after the K-T event .
angiosperms=flowering plants, ungulates=hoofed
animals, macropods= marsupials.
The early Tertiary was warm, but cooled through
the Eocene. By the Pliocene, the world had
significant ice. By the Pleistocene, there were
frequent advances and retreats of ice across
Europe and North America.
The pulse-like ice expansions (glacials)
and contractions (interglacials) seem to
have been driven by Milankovitch
Effects.
As climate cooled, tropical rainforests
contracted and were replaced by
grasslands and deserts in the tropical
zone, and the biomes familiar to us –
prairies, boreal forest, temperate
broadleaved forests - formed. Until then,
much of North America would have been
forested. Until the mid-Tertiary, these
forests extended across the Arctic
archipelago!
Present global
biomes(left) and in
the Eocene
(below)
Tertiary fossil forests
in the Canadian
Arctic.
One order that evolved was the Primate Order. We evolved from these as the family Hominideae, in the genus Homo. Our species? Homo sapiens.
This evolutionary process appears to have been largely driven by the global climatic changes from the mid-Tertiary.
As climate cooled, tropical rainforests contracted and were replaced by grasslands and deserts in the tropical zone. Until the mid-Tertiary, much of the Arctic archipelago had forest cover.
Currently there
are 16 families,
72 genera and
about 350
species of
primate.
Evolution of primates.
Our ancestors, the primates, were once widely
distributed, but they have always been largely
tropical fauna and largely arboreal.
Thus, climatic cooling caused their contraction
into the current tropical zone and the loss of
forest cover stimulated some to become ground
dwellers. This required the exploitation of
different resources so there were morphological
and behavioural changes. The most obvious of
the latter was the development of bipedalism.
Current distribution of primates
Our lineage
Early primate
Ring-tailed lemurs. Lemurs are
confined to Madagascar. Why?
Old and New
World
monkeys.
We share about 96% of our genes
with gorillas
And 98% with chimpanzees
Our African Origins;
Although climatic changes through
the Tertiary were global, the evolution
of hominids/hominins was not . It was
confined to Africa. Was this because
Africa had a suite of environmental
conditions that only allowed our
evolution to occur there or was it
fortuitous?
Why Africa?
(a) increasing aridity and seasonality
leads to major contraction of once
extensive rainforest
(b) expansion of tropical savanna
(c) creation of isolated forest fragments
in a ‘sea’ of savanna
(d) isolation facilitated by uplift and
volcanism in the Rift Valley system
(e) strong seasonality with fire a natural
part of the cycle, and long animal
migrations.
(f) strong coevolutionary associations
between herbivores, carnivores and
scavengers.
Massive environmental changes then triggered life-style and dietary changes in hominids. Obvious consequences included bipedalism and an increase in body (and brain) size.
These responses are evident with both
proto humans and all species of Homo.
The proto humans are usually considered
to be the Australopithecenes. A.
afarensis , Lucy, is the best known of
these. A new genus, Ardipithecus, was
recently recognized. It predates the
Australopithecines.
About 2.6 MYBP the first species of Homo
(H. habilis) appeared, then H. erectus.
Bipedalism; Bipedalism may be seen as a
response to changing lifestyle; from
being arboreal to a life on the ground.
Benefits – increasing mobility,
increasing travel range, arms no
longer required for locomotion,
height gain, tool preparation,
thermoregulation, etc.
Changes from knuckle-walking to
full bipedalism
Increasing Size;
Increasing size was associated with broadened dietary niche, increased home range, increased mobility, change in scavenging activities, increase in thermoregulatory efficiency, increased longevity, slower reproductive rates, increasing brain size and complexity, increasing sociality.
Increasing
brain size and
complexity
Is there a correlation between
brain size and environmental
stress?
Humans appear to be confined
to Africa until the exodus of
Homo erectus about 1.7
million years ago.
In the light of recent
discoveries, what happened
after that is becoming more
murky than it was.
Some wrenches in the works;
The Jebel Irhoud site in Morocco –H.
sapiens here at 300,000 YBP ?
The Misliya site in Israel – 200,000
YBP ?
H. erectus sites in Britain – 1 MYBP?
Happisburgh site, Norfolk, UK
Our global diaspora.
Two main models have been proposed to explain the spread of humans around the globe;
1. The Out of Africa Model – by which human evolved in Africa then spread to other parts of the world (twice!).
2. The Multiregional Model – initial spread by H. erectus, then regional evolution of various strains of H. sapiens.
Both models propose an initial migration by Homo erectus around 1.7 million years ago, but they differ with what ensues.
The Out of Africa model is the most popular, but on the basis of recent discoveries, it appears to be far too simple and may also require rethinking of H. sapiens’ origins.
Homo erectus and the first
Out of Africa exodus.
H. sapiens already Out of Africa?
A second exodus and
interbreeding with H. erectus?
Consolidation and further
expansion?
The later migrations
So modern humans evolved earlier than
previously thought.
‘Where’ is now not obvious.
They spread probably at least 100,000
years earlier than previous estimates.
There were probably several exoduses.
They interbred with species evolved from
the much earlier Homo erectus
movement out of Africa.
The role of climate?
Climate provides an envelope in which
biota evolve, exist and expire.
Changing climate undoubtedly
constrained the evolution of hominins,
their geography and their global
expansion.
Much of that evolution took place during
the ice advances and retreats of the
Quaternary.
Glacial advances shaped their movements.
Ice impeded movement in Europe and determined dispersal routes in North America.
At full glacials, sea level was 120 m below current levels. The resulting land connections facilitated dispersal from Africa, across the Near East ,through southeast Asia to Australia and from Siberia into Alaska (Beringia).
Sea level at maximum glaciation –
120 m below current sea level.
Beringia and
human entry to
the Americas.
Late Pleistocene megafaunal
extinctions
The early migrations did not seem to cause any obvious environmental impacts, but the later ones (to Australia and the Americas ) are marked by a massive die off of large animals (megafauna). In North and South America, 75-80% of big animals became extinct. In Australia, over 80%.
North American Megafauna
Australian Megafauna
The coincidence of these extinctions with the arrival of humans on those continents suggests an anthropogenic cause, but the events occurred in times of rapid and large climatic shift. Climatic change is likely to have been at least a contributing factor.
Extinction of Irish Elk and Mammoth
in Eurasia
Pattern of Extinction in North
America
Cave Art – Lascaux, France.
Australian
aboriginal rock art.
How could small populations
with limited technology
eliminate large numbers of big
animals?
Why didn’t the extinction of
large animals happen in Africa
and Eurasia?
So, the first major environmental impact
of humans may have been the mass
extinction event in the Late
Pleistocene/Early Holocene.
It’s also from this time that we have the
first evidence of plant and animal
domestication.
Domestication brought food surpluses,
allowed population increase,
urbanization, stratification of society –
the emergence of civilization.
We were going to look at plant
and animal domestication and
its consequences next week,
but because of scheduling
issues it will wait until Feb. 16.
Instead, we’ll talk about El Nino
(ENSO) and its impacts on past
and present societies.