1- Chapter 23: human evolution notes

Core Concepts

Anatomical, molecular, and fossil evidence show that the human lineage branches off the tree of great apes.
According to phylogenetic analysis of mitochondrial and Y-chromosome DNA sequences, our species arose in Africa.
The human lineage acquired a number of distinctive features in the $5\text{--}7\ \text{million years}$ since the most recent common ancestor of humans and chimpanzees.
Human history has had an important impact on patterns of genetic variation in our species.
Culture, language, and consciousness are developed to a remarkable degree in humans.

The Great Apes: our place in the lineage

Three pools of information inform systematics: fossil record, DNA (molecules), and anatomy.
Core Concept: Anatomical, molecular, and fossil evidence shows that the human lineage branches off the great apes tree.

Primate family tree

Focus on the Apes and the human lineage within the primates.

Ape family tree: physical features (anatomy) and DNA

Human and chimp DNA can be mixed to form hybrids; denaturation occurs at $93^{\circ}$ , one degree less than human–human DNAs.
This temperature difference corresponds to about a $1\%$ difference between the two sequences.
The members of all the different species in the lineage leading to humans are called hominins.
Earliest known hominin: $\text{Sahelanthropus tchadensis}$ (~ $7\text{ million years ago}$ ) from Chad; chimp-sized brain but hominin brow ridges, indicating early split from the shared ancestor with chimps.

The earliest hominin: Sahelanthropus tchadensis

discovered in 2002 in Chad; dated to be about $7\text{ million years ago}$ ; combines modern human and ancestral features; chimp-sized brain but hominin brow ridges.

Did modern humans evolve from chimps?

No. Modern humans evolved from the same common ancestor as chimps; chimpanzees and humans have never been the same species with one branching off from another.
Comparison: frogs vs. salamanders, mice vs. rabbits illustrate separate lineages since common ancestry.

Lucy (Australopithecus afarensis) and Ardi (Ardipithecus ramidus)

Hominins have been walking upright for at least $3.2\ ext{million years}$ .
Ardipithecus ramidus (Ardi) was $4.4\ ext{million years}$ ago, bipedal on land but quadrupedal in trees.
Somewhere between Ardi and Lucy (approx. $1.2\ ext{million years}$ apart), hominins became fully bipedal.

Hominin Lineages: Genus Homo

The hominin lineage produced many species in Africa; as many as three Homo species may have lived simultaneously.
All hominins share a common ancestor; not all Homo lineages lead to modern humans.
There were multiple extinctions of Homo lineages besides the line leading to Homo sapiens.

Trends in brain and body size

Cranial size increased over time; some brain size increase scales with body size.
In hominins, brain size increased much faster than body size.

Core concepts (recap)

1) Anatomical, molecular, and fossil evidence show that the human lineage branches off the tree of great apes.
2) Our species arose in Africa according to mtDNA and Y-chromosome analyses.
3) The human lineage acquired distinctive features in the $5\text{--}7\ \,\text{million years}$ after diverging from chimpanzees.
4) Human history has shaped genetic variation patterns in our species.
5) Culture, language, and consciousness are highly developed in humans.

Human migratory routes impacted distribution of genetic variation

Movement patterns of early humans influenced how genetic variation is distributed globally.

Neanderthals and modern humans: timelines

Neanderthals: $6\times 10^{5}$ to $3\times 10^{4}$ years ago.
Modern humans: $3\times 10^{5}$ years ago to today.

Tree of recent hominins

Overview of lineages from late archaic humans to modern humans.

The Multiregional Hypothesis

Proposes that modern humans derive from Homo ergaster populations that spread around the world starting about $2\ times\ 10^{6}$ years ago.
Implies parallel evolution of modern human traits with limited gene flow between regions; convergent evolution in multiple populations.

The Out-of-Africa Hypothesis

Developed in 1987; suggests modern humans arose much more recently from Homo ergaster descendants (often called Homo heidelbergensis) in Africa, about $2\times 10^{5}$ years ago (roughly $200{,}000$ years ago).
Implies a single evolutionary origin for modern human traits.

Using mtDNA to test hypotheses on human origins

Constructing a phylogenetic tree from restriction sites.
mtDNA data support Out-of-Africa; Y-chromosome data provide similar support.
In sum: we are a young species that originated in Africa.

Core concepts (final recap)

Re-listing core concepts for quick recall:
1) Anatomical, molecular, and fossil evidence show that the human lineage branches off the tree of great apes.
2) Our species arose in Africa according to mtDNA and Y-chromosome evidence.
3) The human lineage acquired distinctive features in the $5\text{--}7\ \text{million years}$ after the MRCA with chimpanzees.
4) Human history has shaped genetic variation in our species.
5) Culture, language, and consciousness are highly developed in humans.

Language and conscious thought

Humans exhibit advanced language and conscious thought beyond other species.

Final takeaway

Neanderthals and modern humans share a recent common ancestor with early Homo species.
Modern humans originated in Africa and dispersed globally, replacing or contributing to regional populations.
The integration of fossil, anatomical, and molecular data supports a recent African origin for Homo sapiens and highlights rapid brain evolution and neotenous features in our lineage.

References to figures (context)

Figures and captions referenced (e.g., Lucy, Ardi, Neanderthal timelines) support these core ideas and timelines.