The fossil record and common ancestry

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Lesson notes

Fossil Record

The **fossil record** shows the history of life on Earth, including when organisms existed, their diversity, and when they went extinct.
Fossils are preserved remains or traces of ancient organisms, usually found in sedimentary rock layers.
Older fossils are found in deeper rock layers; younger fossils are found in layers closer to the surface.
The fossil record provides evidence of **change over time**—many species that once lived are now extinct, and new species appear in younger layers.
Transitional fossils, like×Tiktaalik×(fish to tetrapod), show intermediate forms between ancestral and descendant groups.

Common Ancestry

**Common ancestry** means that different species share a common ancestor from which they evolved.
All life on Earth is thought to have descended from a **last universal common ancestor (LUCA)** that lived about 3.9 billion years ago.
The more recent the common ancestor, the more closely related two species are.
Charles Darwin proposed universal common descent in×On the Origin of Species×(1859).

Evidence from Anatomy

**Homologous structures** are body parts in different species that share a common evolutionary origin, even if they have different functions (e.g., human arm, whale flipper, bat wing).
Homologous structures indicate that species inherited the structure from a common ancestor and it was modified over time.
**Analogous structures** have similar functions but different evolutionary origins (e.g., bird wing and insect wing); they do not indicate common ancestry.
Vestigial structures are remnants of organs that had a function in ancestors but are reduced or nonfunctional today (e.g., human appendix, whale pelvis).

Evidence from Embryology

**Embryological similarities** among vertebrates (e.g., fish, amphibians, reptiles, birds, mammals) suggest a common ancestor.
Early embryos of different species often have structures like pharyngeal pouches and a tail, which later develop into different adult forms.
These shared embryonic features provide evidence that all vertebrates evolved from a common ancestor.

Evidence from Biochemistry and Genetics

All known life uses **DNA** to store genetic information and **RNA** and **ribosomes** to make proteins, indicating a common origin.
The **genetic code** (mapping of codons to amino acids) is nearly universal across all organisms.
Comparing DNA sequences shows that more closely related species have more similar DNA (e.g., humans and chimpanzees share ~98% of their DNA).
Proteins like **cytochrome c** are found in all organisms; small differences in their amino acid sequences reflect evolutionary relationships.

Key Concepts

**Speciation** is the process by which one ancestral species splits into two or more descendant species.
A **phylogenetic tree** (or evolutionary tree) diagrams the evolutionary relationships among species based on common ancestry.
The fossil record, anatomy, embryology, and genetics all provide converging lines of evidence for evolution and common descent.
No reliable observation has ever contradicted the general notion of common descent; it is a foundational fact in biology.

Homologous forelimb bones in humans, whales, bats, and birds indicate descent from a common ancestor.

DNA double helix: all life uses DNA to store genetic information, supporting common ancestry.

Energy pyramid showing trophic levels; not directly about common ancestry, but illustrates ecological relationships.

Slides

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Practice questions

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1.What does the fossil record provide evidence for?
Easy
- AChanges in species over time and common ancestry
- BThat all species were created at the same time
- CThat fossils are the remains of modern organisms
- DThat evolution occurs only in plants
2.Which of the following is the best evidence that two species share a common ancestor?
Easy
- AThey live in the same habitat
- BThey have similar DNA sequences
- CThey eat the same food
- DThey are the same size
3.The diagram shows a fossil of a trilobite. Which statement about trilobites is supported by the fossil record?
Medium
- AThey are still alive today
- BThey lived only in the oceans
- CThey lived millions of years ago and are now extinct
- DThey evolved from mammals
4.Which of the following is NOT a type of evidence used to support common ancestry?
Medium
- AFossil record
- BComparative anatomy
- CSimilarities in embryonic development
- DThe color of an organism's fur
5.A scientist finds fossils of a species in lower rock layers and similar but more complex fossils in higher layers. What does this pattern suggest?
Hard
- AThe species evolved from a common ancestor over time
- BThe species migrated from one place to another
- CThe fossils are from different species that are unrelated
- DThe rock layers have been disturbed by earthquakes
6.What is the name of the hypothetical common ancestor of all living organisms?
Easy
- ALUCA
- BDarwin
- CE. coli
- DEukaryote
7.Which of the following is an example of a homologous structure that provides evidence for common ancestry?
Medium
- AThe wing of a bird and the wing of a butterfly
- BThe arm of a human and the flipper of a whale
- CThe eye of an octopus and the eye of a human
- DThe shell of a turtle and the shell of a crab
8.The fossil record shows that horses have evolved from small, multi-toed ancestors to large, single-toed modern horses. This is an example of:
Hard
- AGradualism
- BPunctuated equilibrium
- CCatastrophism
- DUniformitarianism

Revision notes

## The Fossil Record and Common Ancestry - **Fossil record** shows patterns of existence, diversity, extinction, and change over time. - **Common ancestry** means species share a common ancestor. The more recent the common ancestor, the more closely related the species. - All life on Earth likely descended from a **last universal common ancestor (LUCA)** ~3.9 billion years ago. - Evidence for common ancestry includes: - **Fossils**: show gradual change and transitional forms. - **Anatomical similarities**: homologous structures (same basic form, different functions) suggest shared ancestry. - **Embryological similarities**: embryos of related species look alike. - **Biochemical evidence**: all organisms use DNA, RNA, ribosomes, ATP, etc. Some 23 proteins are found in all organisms; 6,331 genes are common to all animals. - Charles Darwin proposed universal common descent in *On the Origin of Species* (1859). - The fossil record is incomplete, but no reliable observation contradicts common descent. ## Key Terms

fossil record: The collection of all known fossils, showing patterns of life's existence, diversity, extinction, and change over geological time.
common ancestry: The concept that two or more species share a common ancestor; the more recent the common ancestor, the more closely related the species.
last universal common ancestor (LUCA): The most recent common ancestor of all currently living organisms, estimated to have lived about 3.9 billion years ago.
homologous structures: Anatomical features in different species that have the same basic form but may serve different functions, indicating shared ancestry.
embryological similarities: Similarities in the early developmental stages of different organisms, providing evidence for common ancestry.

Flashcards

What does the fossil record show about life on Earth over time?

The fossil record shows that life has existed, changed, and gone extinct over millions of years, with new species appearing and others disappearing.

How do fossils provide evidence for common ancestry?

Fossils show patterns of change over time, with older rocks containing simpler life forms and younger rocks containing more complex forms, suggesting descent from common ancestors.

What is the last universal common ancestor (LUCA)?

LUCA is the most recent common ancestor of all currently living organisms, estimated to have lived about 3.9 billion years ago.

What are two early pieces of evidence for life on Earth?

Biogenic graphite in 3.7 billion-year-old rocks from Greenland and microbial mat fossils in 3.48 billion-year-old sandstone from Australia.

How many groups of genes are common to all living animals?

6,331 groups of genes are common to all living animals, likely from a common ancestor that lived 650 million years ago.

Who first proposed universal common descent through evolution?

Charles Darwin in his 1859 book On the Origin of Species.

What did Darwin mean by 'from so simple a beginning endless forms most beautiful and most wonderful have been evolved'?

He meant that all diverse life forms evolved from one or a few simple ancestors through natural processes over long periods.

What is the idea that all living things are related called?

Common descent or universal common ancestry.

What evidence from biochemistry supports common ancestry?

All known life uses DNA for genetic information, RNA for transcription, and ribosomes for protein synthesis, with shared molecules like ATP for energy.

What is cytochrome c and how does it support common descent?

Cytochrome c is a protein found in many organisms; small differences in its sequence across species reflect evolutionary relationships, supporting common ancestry.

How many proteins are found in all organisms?

About 23 proteins are found in all organisms, serving core functions like DNA replication, indicating a single ancestry.

What does the similarity of embryos across different species suggest?

Embryological similarities, such as early stages of vertebrates having gill slits and tails, suggest they share a common ancestor.

What are homologous structures?

Homologous structures are body parts in different species that have similar underlying anatomy, indicating they evolved from a common ancestor.

Give an example of homologous structures in vertebrates.

The forelimbs of humans, cats, whales, and bats have similar bone arrangements (humerus, radius, ulna) despite different functions, showing common ancestry.

What is the difference between homologous and analogous structures?

Homologous structures share a common evolutionary origin; analogous structures serve similar functions but evolved independently (e.g., bird wings and insect wings).

How does the fossil record show extinction?

Many fossils of species that no longer exist today, like dinosaurs and trilobites, provide evidence of extinction events.

What does the pattern of fossils in rock layers indicate?

Older rock layers contain simpler fossils, and younger layers contain more complex fossils, showing a progression of life over time.

What is a transitional fossil?

A transitional fossil shows traits of two different groups, providing evidence for evolutionary change between ancestral and descendant species.

Name a famous transitional fossil.

Tiktaalik is a transitional fossil between fish and tetrapods, with both fish-like and land animal features.

How do vestigial structures support common ancestry?

Vestigial structures are remnants of organs that had a function in ancestors but are reduced or useless now, like the human appendix or whale pelvic bones.

What does the genetic code's universality suggest?

All organisms use the same genetic code (DNA to RNA to protein), strongly suggesting they share a common ancestor.

What is the significance of the 6,331 genes common to all animals?

These genes are shared across all animal species, indicating they were present in a single common ancestor that lived about 650 million years ago.

How does DNA comparison help determine common ancestry?

By comparing DNA sequences, scientists can measure how closely species are related; more similar DNA indicates a more recent common ancestor.

What is the role of natural selection in common descent?

Natural selection acts on variations, causing populations to adapt and diverge over time, leading to new species from a common ancestor.

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