Author : Wahid Ahmad
The time
period Between 770,000 and 126,000 years ago, Earth was buzzing with action.
This period called Middle Pleistocene, was a pivotal era in human history. It
witnessed the rise of various human species across different parts of the
world. Modern humans emerged in Africa, while Neanderthals made their mark in
Europe, and the Denisovans left their traces in mainland Asia.
Alongside
advanced species, more primitive features were seen in Homo erectus, Homo
naledi, and Homo floresiensis. Earlier species like Homo antecessor in Spain
and Homo Luzonensis in the Philippines added to the diversity.
During the Middle Pleistocene, significant
changes in human morphology, behavior, and technology occurred. Brain volume
increased, sharp Levallois tools emerged, and evidence of fire use and complex
settlements reflected enhanced cognitive abilities. This era was influenced by
the "Early to Middle Pleistocene transition," marked by global
climatic fluctuations impacting ecosystems, human populations, and their
evolutionary trajectory.
Ancient
DNA studies show insights into relationships between Neanderthals, Denisovans,
and modern humans. Modern humans diverged from Neanderthals around 500,000
years ago and Denisovans around 600,000 years ago. These findings illustrate
diverse evolutionary paths taken by human groups over hundreds of thousands of
years, highlighting the dynamic nature of human evolution worldwide.
Between
approximately 600,000 and 300,000 years ago, different populations of a common
ancestor began to show variations, leading to the formation of distinct groups.
These
groups or subspecies, were geographically separated but still belonged to the
same ancestral species.
After 300,000 years ago, new species emerged
from these differentiated populations. These new species were defined based on
genetic, morphological, and behavioral differences, known as apomorphies.
There's
ongoing debate about how these speciation events occurred and the taxonomic
status of many fossil specimens found in Africa and Eurasia.
The most
curious question is the exact identity of the Last Common Ancestor from which
modern humans, Neanderthals, and Denisovans ultimately originated.
Most scholars identify Homo heidelbergensis
as the Last Common Ancestor based on fossil evidence. H. heidelbergensis
existed between approximately 900,000 and 300,000 years ago and exhibited
significant variability in morphology and geography.
Some propose H. antecessor as a potential Last Common
Ancestor, mainly based on the modern-like facial features observed in Homo
Antecessor found in Spain.
In 1907,
a jawbone was found near Heidelberg, Germany, in the sands deposited by the
Neckar River, the species was named Homo heidelbergensis. Popularly known as
the ‘Mauer jaw’, it was well-preserved, lacking a bony chin, but with teeth
similar to those of modern humans.
Primitive
features in the specimen and suggested that Homo heidelbergensis could be
distinguished from Asian and African representatives of Homo erectus. The Mauer
jaw is often grouped with mandibles and partial craniums from Arago Cave in
France and Petralona in Greece. If we include the Sima de los Huesos in Spain,
additional skulls and numerous postcranial bones can be added to the species
hypodigm.
The
Petralona and Arago specimens also share anatomical similarities with crania
from various locations, including Broken Hill (Kabwe) in Zambia, Elandsfontein
in South Africa, and Bodo from the Middle Awash of Ethiopia. This suggests that
Homo heidelbergensis may have inhabited both Africa and Europe. The species is
considered potentially related not only to Neanderthals but also to modern
humans.
Anatomically,
Homo heidelbergensis had a bigger brain
compared to Homo erectus, averaging around 1,230 cubic centimeters. The skull relatively
is taller than in Homo erectus.
The front of the head is wider, the sides are high
and arched like in modern humans, and the back is more rounded. The part at the
back of the skull is longer than in Homo erectus.
The jaw joint is similar to that of Homo sapiens,
along with certain parts of the ear bone.
The bony ridge above the eyes in is still quite large.
The front part of the head is flatter, like in Homo
erectus. The overall shape is less round compared to modern humans. The face is
connected to the skull in a way that emphasizes the projection of the face.
The nose opening is more vertical, and the canal in
the front of the upper jaw is placed more forward than in Homo erectus.
The point where the two halves of the jaw meet is
receding, and there's a developed internal area for the teeth. European jaws of
Homo heidelbergensis also look similar to Neanderthals.
Some individuals from Sima de los Huesos were tall
and robust, with limb bones and a pelvis suggesting possible differences in
size between males and females, similar to what we see in recent populations.
The
classification of Homo heidelbergensis in the human family tree is a debated
topic. The Mauer mandible, dating back 609,000 years, is among the oldest
Middle Pleistocene fossils in Europe, aligning with findings in Bodo, Africa. Some
scientists suggest replacing "Homo heidelbergensis" with "Homo
bodoensis." Supporters say it clarifies Middle Pleistocene hominin
classification, especially from Africa and the eastern Mediterranean. Opponents
argue against changing long-standing names, expressing concerns about confusion
and potential political motivations.
The
evolution and classification of these middle Pleistocene hominins is debated. One
view groups populations into evolving grades within Homo sapiens, indicating a
single lineage with brain and skull advancements. Alternatively, some propose
cladogenesis within Homo, arguing for a unique morphology in living humans that
separates them from most fossil hominins. They believe only those sharing
specific features should be considered Homo sapiens.
Some
researchers propose two lineages. The European branch, including Neanderthals,
traces back to the Middle Pleistocene through fossils like Petralona, Arago,
Sima de los Huesos, and Mauer. This perspective suggests repeated speciation
events.
Another
lineage, rooted in Africa, includes specimens like Broken Hill, Elandsfontein,
and Bodo, referred to as Homo rhodesiensis. Some late members display a mix of
archaic and modern features.
An
alternative view argues minor morphological differences in ancient European and
African fossils, attributing them to geography and intragroup variation. This
approach advocates classifying them under one taxon, Homo heidelbergensis,
challenging distinct lineages.
If the
broader Homo heidelbergensis classification is accepted, it could be the
ancestor to Neanderthals and modern Homo sapiens. Determining the preferred
scenario depends on the strength of morphometric similarities. Notably,
findings from the Petralona cranium support Homo heidelbergensis as a
widespread, polytypic species.
Around
1,000 to 900 thousand years ago, Earth's climate experienced glacial cycles
every 100 thousand years. At the beginning of the Middle Pleistocene, there was
a notable change in the environment, marked by the disappearance of
‘Villafranchian faunas,’ leading to a decline or disappearance of human
populations using Oldowan tools.
After
about 650 thousand years ago, there was a shift in Europe, we start seeing the
use of more advanced Acheulean tools associated with human remains. The tools
and fossils indicate similarities with African contexts. It suggests a possible
migration route from Africa to Europe through the Middle East.
In West
Asia, Acheulean technologies, possibly originating in Africa, appeared before
1.35 million years ago (Ubeidiya, Israel) and from 750 thousand years ago
onwards (Gesher Benot Ya’aqov, Israel). This aligns with archaeological sites
and human fossils in mid-low latitude regions of Africa and Europe. Fossils
from Mauer in Germany and Arago cave in France, distinct from H. antecessor,
resemble those from Ternifine in Algeria, suggesting Homo heidelbergensis
spread from Africa to Eurasia, introducing Acheulean artifacts.
These
findings, including tools and human remains in Italy and Friuli, indicate
advancement, possibly influenced by Africa through the West Asia. After
reaching Europe in the Middle Pleistocene, Homo heidelbergensis adapted to
harsh conditions during glacial cycles, leading to Neanderthal characteristics
through genetic drift in small or isolated groups.
Concurrently,
Homo heidelbergensis in Africa evolved toward anatomically modern Homo sapiens.
Fossils from locations like Irhoud in Morocco, Omo KHS in Ethiopia, Herto in
the Middle Awash region, and Laetoli in Tanzania document this progression
towards Homo sapiens.
Scientists are debating recent findings at Sima de los Huesos,
challenging our understanding of Neanderthal evolution. The skulls from Sima de
los Huesos exhibit traits expected in early Neanderthals, especially SH 5,
which resembles Neanderthal features. Characteristics like a bony ridge above
the eyes, broad nasal bones, and a distinct part at the back of the head
suggest similarities to Neanderthals. Teeth from Sima de los Huesos closely
resemble Neanderthal teeth. Compared to other ancient skulls like Mauer or
Arago, Sima de los Huesos skulls appear more Neanderthal-like. Some scientists
propose Sima de los Huesos as the source population for Neanderthals, while
Mauer and Arago represent a different group. This suggests two coexisting
species during the European Middle Pleistocene: Homo heidelbergensis predating
Neanderthals and early Neanderthals, challenging previous notions of human
evolution complexity. Fossils across Europe hint that Neanderthal traits showed
up first in the Western and Atlantic regions and spread eastward later.
Genetic signals also complicate things. Mitochondrial DNA
suggests a link to Denisovans, while nuclear DNA shows closer ties to
Neanderthals. This makes it hard to place Sima de los Huesos with Neanderthals,
especially with Denisovans in the mix.
Homo sapiens originated in Africa during the late
Middle Pleistocene, potentially evolving from Homo heidelbergensis due to
climate shifts. Following Homo erectus, a gap in fossil records spans 900 to
600 thousand years ago, with advanced features emerging around 600 thousand
years ago, in south-eastern Africa. Further advancements, including
larger brains and modern features, occurred around three hundred thousand years ago.