Conditions for First Life on Earth

• Earth is unique in its ability to host diverse life, which has evolved to utilize the planet's resources.
• Evolution guides life's innovations.

Conditions for First Life

• The origin of life is a critical, planet-changing event.
• Ideal conditions for life's start are explained through theories based on scientific observation.
• Early Earth experienced extreme heat from meteorite bombardment during the Hadean eon.
• Some studies suggest that only a fraction of the crust melted during the Late Heavy Bombardment, leaving refuge areas in the crust or hydrothermal vents.
• Oceans are thought to have developed by 4.3 billion years ago during the early Hadean.
• Scientists search for first life evidence in Archean rocks.
• Discovering definitive evidence of early life forms is challenging due to their microscopic size, lack of hard parts, and the effects of tectonics and weathering.
• The oldest known fossils were proposed to be in the 3.5 billion-year-old Apex Chert of western Australia, but this is debated.
• Sulfur-metabolizing microfossils in the 3.4 billion-year-old hydrothermal cherts of the Strelley Pool Formation in western Australia are considered the oldest definitive organic microfossils.
• Tubular and filamentous structures near hydrothermal vent deposits of the 3.77-4.28 billion year old rocks of the Nuvvuagittuq Belt in Quebec, Canada, also show evidence of early life.
• Early life is thought to have begun in warm, chemically enriched hydrothermal vent environments.
• Hydrothermal vents are found in the deep ocean along mid-oceanic ridges and at volcanic hot spots.
• Cold ocean water seeps into fissures at plate boundaries, heats up, and mixes with fluids from rising magma.
• Super-heated water carries dissolved minerals and ions, which precipitate as mineral deposits around the vents.
• Sulfur-based minerals are common near vents, and sulfur-reducing bacteria extract energy from compounds in the vented fluids.
• The first organisms at Hadean hydrothermal vents were likely sulfur-reducing bacteria, which are chemoautotrophs.
• Chemoautotrophs get energy from inorganic compounds.
• Sulfur-reducing bacteria "breathe" sulfate (SO42-) and reduce sulfur to hydrogen sulfide (H2S).
• Sulfur-reducing bacteria are found at the Mid-Atlantic Ridge, East Pacific Rise, and in Yellowstone National Park's hot springs and geysers.
• Some sulfur-reducing bacteria are also found in coastal salt marshes, producing a rotten-egg odor.
• These prokaryotes belong to the Domain Archaea and can live in chemically harsh conditions over a wide range of temperatures.

The Prokaryotic Pioneers

• The first life forms on Earth were likely sulfur-reducing bacteria, which are prokaryotes.
• Prokaryotes are simple, single-celled organisms (0.1 – 10.0 μm) without a nucleus or organelles.
• Prokaryotes have significantly changed Earth's history.
• Two of the three Domains of life, Archaea and Bacteria, are prokaryotes.
• Eukarya is the third Domain.
• Prokaryotes live in diverse environments, including volcanic lakes, hypersaline springs, glacial ice, caves, and mountains.
• They outnumber human cells in the human body by 3 to 1.
• There are an estimated 2 million species of oceanic prokaryotes.
• Prokaryotes have been evolving for 3.5 billion years.
• Stromatolites, layered structures created by cyanobacteria, are found in rocks of similar age to the Archean sulfur-reducing bacteria.
• Cyanobacteria mats are smothered by fine calcite mud in shallow marine environments, creating stromatolites.
• The most widely accepted definitive stromatolites are those in the 3.5 billion year old Dresser Formation in Australia.
• Cyanobacteria are photoautotrophs, requiring sunlight for photosynthesis.
• Photoautotrophs are found in the shallow, photic zone of marine environments.
• Photosynthesis uses carbon dioxide (CO2) and produces free oxygen (O2).
• Cyanobacteria were the dominant life form for about 2 billion years.
• Cyanobacteria significantly increased the amount of O2 in Earth’s atmosphere, leading to the Great Oxidation Event.
• Banded iron formations (BIFs) first appear around 2.4 billion years ago
• BIFs consist of alternating layers of dark grey to black iron oxides and red to yellow chert.
• Excess oxygen from cyanobacteria combined with iron ions in seawater to produce the dark oxidized iron layers of BIFs.
• Iron came from hydrothermal vents and chemical weathering of mafic-rich crustal rock.
• BIFs were globally pervasive until about 1.8 billion years ago.
• BIFs declined as the amount of oxygen in the atmosphere increased, leading to iron ions oxidizing in terrestrial settings before reaching the oceans.

Eukaryotes: One Small Step for Cells, One Giant Leap for Life

• Life modifies Earth systems, and Earth systems modify life.
• The accumulation of oxygen in the atmosphere created a new energy source for life.
• Definitive eukaryotes appear around 1.8 billion years ago, using aerobic respiration to make energy.
• Aerobic respiration requires oxygen to produce ATP molecules for cellular function.
• Eukaryotic cells have a membrane-bound nucleus containing DNA and other organelles.
• Mitochondria are organelles associated with ATP production in eukaryotic cells.
• Chloroplasts are necessary in plants for photosynthesis.
• Eukaryotic cells are generally 10 – 100 μm in size, larger than prokaryotic cells.
• Endosymbiosis theory (ET) states that eukaryotic cells arose when one prokaryote lived inside another in a mutually beneficial relationship.
• The host cell offered protection, and the ingested cell provided energy.
• Cells co-evolved and became reliant on each other.
• Lynn Margulis championed ET in the 1960s.
• Mitochondria and chloroplasts possess their own DNA similar to modern prokaryotes, supporting ET.
• Organisms without mitochondria host symbiotic bacteria that serve the same function.
• Eukaryotes include single-celled organisms (protists) like amoeba, foraminifera, coccolithophores, radiolaria, and diatoms.
• All multicellular organisms are also eukaryotes.
• The first eukaryotes are thought to be acritarchs, single-celled organisms appearing about 1.8 billion years ago.
• Acritarchs are of unknown affinity.
• Acritarchs are spherical sac-shaped cells, ranging from 1.0 – 1000.0 μm.
• Grypania spiralis, a filamentous, spiral shaped fossil, also appears in the fossil record around 1.8 billion years ago.
• Grypania spiralis is considered to be eukaryotic due to its size.
• Some researchers think Grypania may represent one of the first multicellular organisms.

Out onto the Multicellular Branches of Life: The Ediacaran Biota

• Multicellularity's evolutionary benefit could include increased survival under predation.
• Multicellularity is an example of convergent evolution, occurring multiple times within different eukaryotic groups.
• Sexual reproduction is another potential evolutionary benefit shared by most multicellular organisms.
• Sexual reproduction involves gametes recombining DNA to make genetically distinct offspring.
• Asexual reproduction results in a genetic replicate unless a random mutation occurs.
• Sexual reproduction can more rapidly produce advantageous traits and delete harmful mutations, but takes longer to grow population sizes.
• The Red Queen hypothesis proposes that asexual organisms are more susceptible to parasitism.
• Recombination of DNA allows for genetic variability to defend against co-evolving parasites.
• Both asexual and sexual reproduction are successful means to fully take advantages of Earth’s resources.
• The Ediacaran Period (635-541 million years ago) captures the origins of multicellular life.
• The Ediacaran biota are a diverse assemblage of complex, macroscopic multicellular body and trace fossils.
• This increase in diversity follows an extensive glaciation period (Snowball Earth) during the Cryogenian.
• This biota originated during a time when oxygen levels in the surface oceans were increasing.
• High bacteria levels created higher concentrations of dissolved organic matter for these organisms.
• Warmer, oxygen- and food-rich seas may have set the conditions for multicellular life to flourish.
• Ediacaran fossils have been found on every continent except Antarctica.
• They are typically preserved within sandstones of shallow marine settings.
• Fossils have also been found in finer-grained sediments of deeper waters.
• The biota consists of impressions of soft-bodied organisms.
• Microbial mats would aid in preservation of the fossil impressions.
• Common Ediacaran fossil shapes include disc-shaped forms, frond-like shapes (rangeomorphs), and examples of bilateral symmetry.
• Anatomic detail is hidden due to the coarse sediment in which these fossils are preserved, leading to debate about their placement on the tree of life.
• Possibilities include basal members of invertebrate animal groups, giant protists, algae, worms, fungi, or in a phylum of their own (Vendozoa).
• Recent chemical analyses of Dickinsonia fossils showed evidence of cholesteroids, found only in animal cells.
• Molecular analyses of the surrounding sediments found stigmasteroids, indicative of the green algae of the microbial mats.
• Some Ediacaran biota are some of the first true animals on Earth.
• Most Ediacaran burrows are simple, horizontal trails and burrows, or rasping traces.
• Snails and sea urchins eat algae this way today.
• Burrows increase in complexity toward the end of the Ediacaran, with more vertical and branching shapes, suggesting increasing animal motility.

Phanerozoic Diversity of Life (According to the Hard Parts)

• Diversification of life through time is a fascinating aspect of Earth's history.
• The "tree of life" diagram represents phylogenetic relationships of the three major Domains of life: Archaea, Bacteria, and Eukarya.
• Eukarya splits into the four Kingdoms: Protista, Fungi, Plantae, and Animalia.
• During the Cambrian Period, shelled organisms with an exoskeleton proliferate in the fossil record, known as the Cambrian Explosion.
• Small, shelled animals (metazoans) of uncertain affinities are found in Late Ediacaran rocks, including the Tommotian fauna.

Before and After: The Exoskeleton Appears

• Animals developed mineralized exoskeletons to protect against physical and chemical variations, biological predators, and to allow muscles additional mechanical leverage.
• The boundary between the Proterozoic and Phanerozoic is marked by a shift in seawater chemistry impacting carbonate mineral deposition.
• Calcium influx during the early Cambrian allowed for easier precipitation of aragonite and calcite, used by marine organisms to create shells.
• A wide array of body plans developed among representatives of most of today’s major invertebrate phyla as the Cambrian and Ordovician progressed.
• Internal skeletons (endoskeletons) also originated in the Cambrian Period.
• The first endoskeletons were cartilage.
• The first vertebrates are fossils from the early Cambrian Chengjiang locale in China.
• These vertebrates were marine organisms with a notochord and simple vertebrae.
• A notochord is a skeletal structure shared by all members of the phylum Chordata.
• The notochord supports muscles and defines bilateral symmetry.
• Vertebrae provide additional strength, flexibility and support.
• Jawless fish (agnathans) and armored fish (ostracoderms) diversified throughout the Cambrian and into the Silurian.
• External plates of the ostracoderms provide protection and ion storage (phosphorous) for muscles.
• Jawed fish, including cartilaginous fish and bony fish, appeared during the Silurian and radiated during the Devonian.

Here Come the Plants: Landward, Ho!

• Plants moved to land during the Paleozoic.
• The first multicellular plants are marine green and red algaes, likely derived from photosynthetic cyanobacteria during the Neoproterozoic.
• The first land plants evolved around the early Ordovician: bryophytes, including mosses, hornworts, and liverworts, found in moist areas.
• Bryophytes do not rely on a root system, taking in water and nutrients via their leaves and reproducing via spores.
• The first true vascular plants (trachaeophytes) evolved later in the Ordovician.
• By the Devonian, this group looked like plants we are familiar with – leafy and green, possessing a lignin-reinforced stem and roots, and using seeds for reproduction.
• Plants assist in soil development from bedrock, extract nutrients, retain water, and produce organic matter.
• Vascular plant root systems wedge into cracks in rock, physically creating more surface area for weathering.
• Plants add stability to slopes and provide new resources for other life forms.

Next Up: Invertebrate Animals Invade Terrestrial Environments

• Invertebrate groups diversified in the Paleozoic oceans, increasing competition for resources.
• Ecological tiering increased above and below the seafloor.
• The transition to land occurred during the Devonian period, when global changes in oxygen levels in the ocean were in decline.
• Dysoxia was driven by excess runoff of organic matter from land due to dying plants.
• Dying plants triggered plankton blooms that depleted oxygen in the oceans.
• Aquatic invertebrates had an advantage due to their exoskeletons, adapting to freshwater ecosystems and eventually land.
• Arthropods are the first aquatic invertebrate group to venture into the terrestrial realm.
• Arthropods had a body plan with segmented appendages specialized for different types of movement.
• Aquatic arthropods take in oxygen via book gills.
• The first land arthropods developed internal book lungs from these structures.
• Terrestrial fossils of spider-like arthropods are found from the Late Silurian to Early Devonian, in association with plant fossils, scorpions, and myriapods.
• Some groups of arthropods, including insects, developed tracheae, delivering oxygen from external pores in the exoskeleton to tissues.
• Insects have spread across the terrestrial realm through:
  • Coevolution among insects and with other animals
  • Coevolution with plants
  • Adaptation to freshwater
  • Variable predation in larval versus adult forms
• Arthropods developed wings and flight, first appearing in the fossil record of the Carboniferous.
• Wings represent a modification of appendages given a boost from high oxygen levels in muscles.
• The development of flight corresponds to increases in atmospheric oxygen during the Carboniferous.
• This diversification of insects is known as an adaptive radiation.
• There are about one million described insect species, representing almost 75% of all known animals.

Vertebrates Take Their First Steps

• Vertebrates show increases in eye size and a shift in eye position leading up to their transition to land.
• These eyesight changes allow better vision over longer distances.
• Structural changes were also required to the vertebrate body plan.
• The first land-based animals are known as tetrapods, moving on four feet.
• Tetrapod bones needed to be more robust to provide structural support.
• Bony, lobe-finned fish (sarcopterygian class) possess the basic vertebrate body plan in their appendages.
• Their fins exhibit the same sequence of bones as tetrapod limbs: one bone, a joint, two bones, another joint, then lots of little bones.
• Other changes key to success on land include: development of a three-chambered heart and more complex lungs, modifications to auditory systems, and development of more waterproof skin.
• Paleontologists look for transitional fossils showing these gradual changes over time.
• Tiktaalik possessed stronger limbs, hip-like structures, a neck, and large eye sockets.
• An increase in size and shift in the position of spiracles is seen in several transitional fossils.

Tetrapods and Plants: Conquering Land, Sea, and Air

• The first tetrapods on land during the Devonian evolved characteristics reminiscent of today’s amphibians.
• Amphibians require water for reproduction and have eggs without a protective outer coating.
• During the Carboniferous, tetrapod groups evolved the amniotic egg, which protects the developing embryo with protective layers.
• The amniotic egg allowed tetrapods to spread into different inland environments.
• Amniotes are represented by reptiles (including dinosaurs and birds) and mammals.
• During the Late Carboniferous, two distinct lineages of tetrapods arose: sauropsids and synapsids.
• Sauropsids diversified after the end-Permian extinction and led to reptiles like crocodilians, turtles, snakes, dinosaurs, and birds.
• The end-Cretaceous mass extinction wiped out many reptilian groups, allowing mammals to repopulate ecological niches.
• Synapsids began with a reptilian-like body plan and a single opening in the skull behind the eye socket.
• Changes within this clade led to the various groups of mammals, including monotremes, marsupials, and placental mammals.
• Vascular plants also evolved throughout the Mesozoic.
• Angiosperms evolved during the Cretaceous.
• Angiosperms are the most common plants on Earth today.
• Gymnosperms primarily reproduce via wind-blown pollen released from cones.
• Angiosperm flowers are reproductive structures that allow for pollen distribution by organisms.
• Gymnosperm seeds are "naked seeds" not encased in an ovary.
• Angiosperm seeds are encased in ovaries we know as fruit.
• These relationships between plants, pollinators, and dispersers represent coevolution.
• The interactions of living organisms play a key role in how Earth has changed.
• Interactions with the atmosphere, hydrosphere, and geosphere have shaped the tree of life.