EARLY EARTH AND THE ORIGIN OF LIFE CHAPTER 26 EARLY EARTH AND THE ORIGIN OF LIFE

I. INTRODUCTION TO THE HISTORY OF LIFE LIFE ON EARTH ORIGINATED BETWEEN 3.5 AND 4.0 BILLION YEARS AGO EARTH FORMED 4.5 BILLION YEARS AGO. THE OLDEST AVAILABLE EVIDENCE OF LIFE IS CARBON ISOTOPES OF CELLULAR ORIGIN FOUND IN 3.8-BILLION-YEAR-OLD ROCKS. THE OLDEST FOSSILS ARE 3.5 BILLION YEARS OLD.

B. MAJOR EPISODES IN THE HISTORY OF LIFE: A PREVIEW THROUGHOUT THE APPROXIMATELY 4-BILLION-YEAR TENURE OF LIFE ON EARTH, GEOLOGICAL AND BIOLOGICAL HISTORY HAVE BEEN CLOSELY INTERTWINED.

II. PREBIOTIC CHEMICAL EVOLUTION AND THE ORIGIN OF LIFE THE FIRST CELLS MAY HAVE ORIGINATED BY CHEMICAL EVOLUTION ON A YOUNG EARTH: AN OVERVIEW ONE HYPOTHESIS ABOUT THE ORIGIN OF LIFE IS BASED ON THE CHEMICAL EVOLUTION OF PROTOBIONTS, ABIOTICALLY PRODUCED MOLECULAR DROPLETS WITH DISTINCTIVE CHEMICAL CHARACTERISTICS

ABIOTIC SYNTHESIS OF ORGANIC MOLECULES IN A MODEL SYSTEM

B. ABIOTIC SYNTHESIS OF ORGANIC MONOMERS IS A TESTABLE HYPOTHESIS: SCIENCE AS A PROCESS LABORATORY EXPERIMENTS PERFORMED UNDER CONDITIONS SIMULATING THOSE OF THE PRIMITIVE EARTH HAVE PRODUCED DIVERSE ORGANIC MOLECULES FROM INORGANIC PRECURSORS.

C. LABORATORY SIMULATIONS OF EARLY EARTH CONDITIONS HAVE PRODUCED ORGANIC POLYMERS SMALL ORGANIC MOLECULES POLYMERIZE WHEN THEY ARE CONCENTRATED ON HOT SAND, ROCK, OR CLAY.

C. PROTOBIONTS CAN FORM BY SELF-ASSEMBLY ORGANIC MOLECULES SYNTHESIZED IN THE LABORATORY HAVE SPONTANEOUSLY ASSEMBLED INTO A VARIETY OF DROPLETS-COACERVATES, MICROSPHERES, AND LIPOSOMES WITH SOME OF THE PROPERTIES ASSOCIATED WITH LIFE.

D. RNA WAS PROBABLY THE FIRST GENETIC MATERIAL THE FIRST GENES MAY HAVE BEEN ABIOTICALLY PRODUCED RNA, WHOSE BASE SEQUENCES SERVED AS TEMPLATES FOR BOTH ALIGNMENT OF AMINO ACIDS IN POLYPEPTIDE SYNTHESIS AND ALIGNMENT OF COMPLEMENTARY NUCLEOTIDE BASES IN A PRIMITIVE FORM OF SELF-REPLICATION. ONCE GENETIC INFORMATION BECAME INCORPORATED INSIDE MEMBRANE ENCLOSED COMPARTMENTS, PROTOBIONTS WOULD HAVE ACQUIRED HERITABILITY AND THE ABILITY TO EVOLVE AS UNITS.

ABIOTIC REPLICATION OF RNA

F. DEBATE ABOUT THE ORIGIN OF LIFE ABOUNDS SCIENTISTS CONTINUE TO DEBATE HOW AND WHERE LIFE ORIGINATED. THREE VIEWS EXPLORE ABIOTIC SYNTHESIS OF ORGANIC MOLECULES: IMPORT VIA METEORITES, RNA GENES VERSUS SIMPLER SELF-REPLICATING MOLECULES, AND ORIGINS OF LIFE IN SHALLOW WATER VERSUS DEEP.

III. THE MAJOR LINEAGES OF LIFE ARRANGING THE DIVERSITY OF LIFE INTO THE HIGHEST TAXA IS A WORK IN PROGRESS THE TRADITIONAL KINGDOM SYSTEM CLASSIFIES ORGANISMS AS MONERA (PROKARYOTES), PROTISTA (RELATIVELY SIMPLE EUKARYOTES), PLANTAE, FUNGI, AND ANAMALIA. NEW INFORMATION IS LEADING TO TAXONOMIC ALTERNATIVES TO THE FIVE KINGDOM SYSTEM.