Evolution: Extinction and the Sixth Mass Extinction Guide

The Cycle of Evolution and the Normalcy of Extinction

• Extinction is defined as the termination of a species. It is a normal part of the evolutionary process characterized by a species' birth, its period of existence, and its eventual death.

• Statistical frequency of extinction:

  • Approximately $95\%$ to $99\%$ of all species that have ever inhabited Earth have gone extinct.

  • On average, a species survives for about $4 \text{ million years}$ before dying out.

• The drivers of normal (background) extinction include:

  • Changes in environmental conditions.

  • The emergence of new predators.

  • Climatic shifts.

  • Geologic activities, such as the sudden appearance of mountain ranges.

• The symbiotic balance of life: The extinction of species unable to adapt or compete creates ecological niches and opportunities for new life forms.

Mass Extinctions: Reseting the Playing Field

• A mass extinction occurs when a planet-wide catastrophe kills off many species simultaneously, fundamentally changing the rules of evolution and leveling the playing field.

• Historical frequency: There have been $5$ major mass extinction events in the past $500 \text{ million years}$.

• These events are recorded in the Earth's geological and fossil record.

The Great Permian Extinction: Peter Ward and the Karoo Desert

• Geologist Peter Ward studies the greatest mass extinction in history, which occurred at the end of the Permian period, approximately $250 \text{ million years ago}$.

• The Karoo Desert in South Africa serves as a primary site for researching this event.

• Peter Ward relates the scale of the disaster through a personal anecdote about an abandoned farmhouse graveyard from the late $1890\text{s}$:

  • In a $5\text{-year}$ span, a whole family (mother, father, two sons) died.

  • The epitaph read: "Take my ashes and set them free Earth."

  • Just as those people were wiped off the face of the Earth with little record of the cause, Ward seeks to find what killed the animals of the Karoo hills $250 \text{ million years ago}$.

• Clues from the rock record:

  • Green layered rocks from the Permian contain skeletons and evidence of activity, such as burrows or tunnels made by organisms.

  • Common species found at the South African Museum in Cape Town include:

    • Lystrosaurus: Tiny plant-eating creatures that were the most common in the Permian.

    • Gorgonopsians: Ferocious predators reaching up to $10 \text{ feet}$ in length.

• The Permian-Triassic Boundary:

  • Climbing through the rock strata represents moving through time. Moving up $50 \text{ feet}$ can represent $2,000$ to $5,000 \text{ years}$.

  • The transition to the Triassic period shows a catastrophic drop in life. The bottom beds of the Triassic contain almost zero fossils and no signs of tunnels or burrows. Ward describes this state as "dead."

Causes and Characteristics of the Permian Collapse

• Potential triggers for the Permian extinction include:

  • A possible comet impact.

  • A dramatic rise in global temperature and dropping sea levels.

  • Massive volcanic eruptions depositing $1 \text{ million cubic kilometers}$ of lava.

  • Increased atmospheric carbon dioxide ($CO_2$) resulting in ravaged ecosystems.

• Survival Probability: Ward uses a Russian Roulette metaphor. If a gun had $10$ chambers, you would have to put $9$ bullets in it to represent the survival odds (a $1 \text{ out of 10}$ chance of survival for species).

• The House of Cards Analogy: Species do not die alone. The collapse of one species triggers the collapse of others as each species "props up" another (e.g., as a food source). When enough cards (species) are removed, the entire house (ecosystem) falls down in a snowball effect.

The Dawn of the Dinosaurs and Early Mammals

• Following the Permian mass extinction, the empty world allowed for evolutionary diversification.

• Two lineages survived into the Triassic that had significant consequences for the future:

  1. The lineage leading to the dinosaurs.

  2. The lineage leading to mammals (mammal-like reptiles).

• In the Triassic, these two groups competed "head-to-head." The lineage leading to dinosaurs became the clear winners within roughly $20 \text{ million years}$.

• Characteristics of mammals during the Mesozoic era:

  • They were tiny, shrew-like or rat-like creatures scurrying in the shadows.

  • They remained "second-class citizens" while dinosaurs became large, diverse, and dominant.

The Gobi Desert Expeditions: Roy Chapman Andrews and Michael Novacek

• Roy Chapman Andrews (1930s): Traveled with $6$ motorcars and $100$ camels to Central Asia seeking early human ancestors. Instead, he found $80\text{-million-year-old}$ dinosaur bones, eggs, and tiny mammal fossils.

• Michael Novacek's findings:

  • Mammals were "nugget-sized" during the time of dinosaurs.

  • In a modern expedition, his team found $50$ mammal skulls by lunchtime, matching the entire recovery amount from the previous $7 \text{ decades}$.

  • Early mammals were nocturnal, scavenged, and reproduced quickly.

  • Transitional features found in fossils: Splint-like bones in the pelvic region suggest support for an abdominal cavity or pouch, similar to modern marsupials like opossums or kangaroos. This represents a transition between primitive egg-laying and advanced reproduction.

The KT Event: The End of Dinosaurs and Rise of Mammals

• Occurred approximately $65 \text{ million years ago}$.

• An asteroid larger than Mount Everest hit Earth at $25,000 \text{ mph}$.

• The impact caused the "House of Cards" to collapse again. This time, the dinosaurs fell because they were large, had few places to hide, and could not easily protect their offspring.

• Small mammals survived the destruction and eventually acted as "colonizers."

• Post-KT Timeline:

  • Mammals remained small for a few million years (lag effect).

  • Once the ecosystem recovered, they diversified into almost every large animal niche on Earth.

  • $35 \text{ million years ago}$: First true monkeys and apes evolved.

  • $5$ to $6 \text{ million years ago}$: First human-like primates emerged in Africa, eventually learning to walk upright.

The Sixth Mass Extinction: Human Impact

• Global human population: $6 \text{ billion}$.

• Current extinction rate: Soaring to over $100 \text{ times}$ the normal background rate.

• Scientists characterize humans as the "new asteroid" bringing about the sixth mass extinction.

Case Study: Kaeng Krachan National Park, Thailand

• Alan Rabinowitz and Tony Lynam monitor the park to assess the health of its ecosystem.

• Empty Forest Syndrome: A forest that appears intact on the surface but has lost the natural components (animals) required to maintain energy flow.

• Trophic Structure Ratios:

  • $100 \text{ pounds}$ of plants sustain $10 \text{ pounds}$ of herbivores, which in turn sustain $1 \text{ pound}$ of carnivores.

  • Healthy populations of large carnivores (like tigers) indicate a stable, healthy habitat.

• Rabinowitz's findings in Kaeng Krachan:

  • Ten years ago, the forest was pristine, with signs of elephants, otters, gibbons, and tigers everywhere.

  • Current efforts involve "camera traps" with motion sensors to quantify numbers.

  • Success: Caught images of tigers, leopards, and the Siamese crocodile (thought to be virtually extinct in Thailand for over a decade).

• Challenges to conservation: Vandalism and theft of cameras ($3 \text{ out of 10}$ cameras retrieved; one was stabbed with a knife). Hunting and human encroachment remain the primary threats.

Biological Invaders: Hawaii as a Microcosm

• Hawaii is home to the second-leading cause of extinction: Biological Invaders (foreign species).

• David Burney's Sinkhole Study (Kauai):

  • Acts as a "poor man's time machine," preserving $10,000 \text{ years}$ of mud and fossil records.

  • Polynesians arrived around $600 \text{ AD}$ with rats, dogs, and pigs.

  • Europeans later brought hundreds more species, including goats.

• Contrast in diversity:

  • There were originally $1,000 \text{ native plant species}$ in Hawaii; there are now over $1,000 \text{ naturalized invasive species}$.

• Other invasive examples:

  • Zebra Mussel: Arrived in the USA in $1988$; consumes plankton and clogs pipes. The government has spent over $\$4 \text{ billion}$ to control them.

  • Brown Tree Snake: Originally from New Guinea; arrived in Guam via aircraft/ships during WWII. Reaches lengths of $11 \text{ feet}$. It caused the extinction of $9$ of Guam's $11$ forest bird species. Hawaii uses trained beagles to prevent its establishment there.

• Weed Species: Mobile, adaptable, and flexible organisms that move into new areas and push out native competition. Humans are described as the ultimate weed species.

Fighting Back: Biocontrol in North Dakota

• Leafy Spurge: An invasive weed from Russia brought accidentally by pioneers. It covers over $1/3$ of some ranches ($4,000 \text{ acres}$ owned by Sai Konzon).

• Economic impact: Cattle will not eat it; chemical treatment costs $\$100 \text{ per gallon}$, which covers only $1 \text{ acre}$.

• Evolutionary solution: The Flea Beetle. These beetles eat the roots and crown of the spurge, weakening it allowing mold to enter.

• Efficiency: Releasing $100$ beetles in one year can yield a harvest of $2 \text{ million}$ the next. They are spread via light airplanes to remote infestations.

Questions & Discussion

• Inquiry into causes of Karoo deaths: Rabinowitz asks the headman of a local village about certain animals like the rhinoceros. The headman confirms that while they used to be there, hunters have wiped them out.

• Hunting Investigation: Rabinowitz asks a local fisherman about the decrease in tiger populations and uses of snares. The fisherman claims no hunting occurs, but Rabinowitz notes the truth likely lies somewhere in between, particularly given the stolen cameras.

• Human Presumption: The notes conclude with a citation of Charles Darwin from $1859$, warning against the presumption that we understand the complex contingencies on which the existence of each species depends. The appearance of new forms and the disappearance of old forms are inextricably linked.