Detailed Notes on Geological Time and Common Ancestry

Learning Outcomes

• LO1: Outline major lines of evidence for an ancient Earth.

• LO2: Outline several major lines of evidence that led biologists to common descent.

• LO3: Explain what a phylogeny represents.

Age of the Earth

• Current scientific consensus: The Earth is approximately $4.55 ext{ billion years}$ old.

• Modern statistics:

  • Cambrian Explosion: occurred about $541-485.5 ext{ million years ago}$.

  • Humans and chimpanzees diverged around $6 ext{ million years ago}$.

  • Modern humans estimated to have emerged about $0.2 ext{ million years ago}$ (or $200,000 ext{ years}$).

  • Written human history dates back around $5,000 ext{ years}$.

Visualizing Geological Time

• Geological time is vast, and visualizations help place significant events.

• Representation methods:

  • Countdown timer analogy for scale:

  • Animals appeared around 9 minutes ago on a 1-hour scale.

  • Humans appeared in the last 0.2 seconds of this timer.

  • Horizontal timeline moving from the Earth's origin (4.6 billion years ago) to present.

• Key events:

  • Prokaryotes originated about $3.5 ext{ billion years ago}$.

  • Multicellular eukaryotes emerged approximately $1.5 ext{ billion years ago}$.

  • Land colonization and diversification of life forms occurred over the next billion years.

Historical Perspectives on Earth's Age

• Archbishop James Ussher calculated the Earth's age in the 1600s using biblical genealogies, famously concluding it began in $4004 ext{ BC}$.

• James Hutton (18th century) recognized geological processes and unconformities, laying foundations for modern geology.

• By the 19th century, the age of the Earth was widely accepted among scientists, with various interpretations of religious texts.

Relative Dating Principles (1800-1900)

• Technique principles:

  1. Principle of Superposition: In sedimentary layers, older layers are at the bottom.

  2. Principle of Horizontality: Layers are initially deposited in horizontal sheets.

  3. Principle of Lateral Continuity: Layers extend over wide areas before they are interrupted by erosion or faulting.

• Development of Geological Time Scale based on relative dating of fossils and rock layers.

Radiometric Dating Techniques

• Radioactive decay provides methods for absolute dating:

  • Carbon-14 can date materials up to $50,000 ext{ years}$ (half-life: $5,730 ext{ years}$).

  • Uranium-Lead dating: useful for dating rocks millions to billions of years old (half-life: $4.47 ext{ billion years}$).

Is the Age of the Earth Controversial?

• In scientific communities, the age of the Earth is well-established; however, it can contradict certain cultural or religious beliefs.

Evidence for Common Ancestry

• Darwin's observations pointed toward genealogical relationships among species. Key evidences include morphological similarities, embryonic development, and fossil records.

• Examples of Atavisms/Vestigial Structures:

  • Hind flippers in dolphins and hind limbs found in some species of whales indicate remnants of evolutionary history.

Phylogeny

• Defined as a hypothesis of the evolutionary relationships among various biological species based on their physical and genetic characteristics.

• Phylogenetic trees visualize these relationships and evolutionary pathways.

Additional Resources

• Suggested readings include works by Brent Dalrymple that discuss the age of the Earth and the concept of common ancestry.

Conclusion

Understanding geological time and the evidence for common descent are key steps in grasping the history of life on Earth and the processes that have shaped it over billions of years.