https://www.khanacademy.org/science/ms-biology/x0c5bb03129646fd6:evolution/x0c5bb03129646fd6:the-fossil-record/v/fossils 2023-09-29T18:57:09.081622294Z Fossils Fossils, preserved evidence of ancient organisms, reveal Earth's history. Formed in sedimentary rock, fossils' ages are estimated through layer analysis and radiometric dating. These methods help place fossils in Earth's fossil record, providing a timeline of the evolution of life on Earth. Khan Academy video https://cdn.kastatic.org/ka_thumbnails_cache/b248b2d1-0b4a-4847-b090-81261d97c3d7_1280_720_base.png Fossils Fossils, preserved evidence of ancient organisms, reveal Earth's history. Formed in sedimentary rock, fossils' ages are estimated through layer analysis and radiometric dating. These methods help place fossils in Earth's fossil record, providing a timeline of the evolution of life on Earth. https://cdn.kastatic.org/ka-youtube-converted/-fT57zGsXZ8.mp4/-fT57zGsXZ8.mp4 302 Geologic time scale https://www.khanacademy.org/science/middle-school-earth-and-space-science/x87d03b443efbea0a:the-geosphere/x87d03b443efbea0a:fossils-and-rock-layers/v/fossils-and-rock-layers 2023-08-04T17:42:39.696972153Z Fossils and rock layers Geologists use a geologic timescale to map Earth's 4.6-billion-year history. They study sedimentary rock layers, or strata, and fossils to understand past events. They use the Law of Superposition and the Law of Crosscutting Relationships to determine the relative ages of rock layers. This process, called relative dating, helps them trace the evolution of life on Earth. Khan Academy video https://cdn.kastatic.org/ka_thumbnails_cache/7974ab0f-0fcb-4caf-b76b-e0cf618645a5_1280_720_base.png Fossils and rock layers Geologists use a geologic timescale to map Earth's 4.6-billion-year history. They study sedimentary rock layers, or strata, and fossils to understand past events. They use the Law of Superposition and the Law of Crosscutting Relationships to determine the relative ages of rock layers. This process, called relative dating, helps them trace the evolution of life on Earth. https://cdn.kastatic.org/ka-youtube-converted/3LObb5WxMdw.mp4/3LObb5WxMdw.mp4 275 Geologic time scale https://www.khanacademy.org/science/middle-school-earth-and-space-science/x87d03b443efbea0a:the-geosphere/x87d03b443efbea0a:fossils-and-rock-layers/e/apply-fossils-and-rock-layers 2026-04-27T10:30:47.716252349Z Apply: Fossils and rock layers Apply your understanding of fossils and rock layers in this set of free practice questions aligned to NGSS standards. Lauren Rosebrugh exercise https://www.khanacademy.org/science/middle-school-earth-and-space-science/x87d03b443efbea0a:the-geosphere/x87d03b443efbea0a:fossils-and-rock-layers/a/fossils-and-rock-layers 2026-04-27T10:30:47.716252349Z Key points: Fossils and rock layers Review your understanding of fossils and rock layers with this free article aligned to NGSS standards. article https://www.khanacademy.org/science/strengthened-shs-chemistry-1/x174677b2bfa4bea2:1st-quarter/x174677b2bfa4bea2:applications-of-isotopes/v/fossil-dating-heredity-evolution-biology-khan-academy 2020-09-09T11:53:28Z Fossil dating Let's explore the relative depth and radioactive dating of fossils. Mahesh Shenoy video https://cdn.kastatic.org/googleusercontent/e9qGy9B_MqoU9Jo_3aE0OCRwstVBbZaqeCt-yW4zQ4YxBMQM_OOwrtF0QP1Rnz0n5qoOb-Hj4StDeoPOg0IEpkU Fossil dating Let's explore the relative depth and radioactive dating of fossils. https://cdn.kastatic.org/ka-youtube-converted/G1TlxtJ6pTQ.mp4/G1TlxtJ6pTQ.mp4 555 Geologic time scale https://www.khanacademy.org/math/engageny-precalc/precalc-3/precalc-3c-exp-lop/v/half-life 2021-01-04T18:35:20Z Half-life and carbon dating Carbon dating is a real-life example of a first-order reaction. This video explains half-life in the context of radioactive decay. Sal Khan video https://cdn.kastatic.org/googleusercontent/A1D3qsWagCLFTIIalI5f9lmbc2WgRuxZeHl7C4SaFx0cdCEbZs0ZTrEKIa3N0mypb92VTDNba1yBfX-fXn8C6lg Half-life and carbon dating Carbon dating is a real-life example of a first-order reaction. This video explains half-life in the context of radioactive decay. https://cdn.kastatic.org/ka-youtube-converted/9REPnibO4IQ.mp4/9REPnibO4IQ.mp4 751 Geologic time scale https://www.khanacademy.org/science/cambridge-o-level-physics-cie/x0e04e0cb682fb793:nuclear-physics/x0e04e0cb682fb793:half-life/e/half-life 2026-04-07T23:17:37.123726494Z Half-life (qualitative) Let's solve some problems to better understand the meaning of the term half-life. And remove some possible misconceptions around the same. Aakash exercise https://www.khanacademy.org/science/ap-biology/natural-selection/evidence-for-evolution-ap/v/carbon-14-dating-1 2023-08-07T17:09:37.131449861Z Carbon 14 dating 1 Carbon-14 forms in the atmosphere through cosmic rays interacting with nitrogen-14, creating a small fraction of carbon-14 in living organisms. When an organism dies, its carbon-14 decays back into nitrogen-14 at a known rate, called a half-life (5,730 years). By measuring the remaining carbon-14 in fossils, scientists can estimate their age, helping us understand the history of life on Earth. Sal Khan video https://cdn.kastatic.org/googleusercontent/mPZoGtLX4UzhlYgN7HPDXacSfr7TM7Aeb32qIM-ZLq4Kd1OSbizg203sheTJuON9lPx8XyjRMIRoXRqOKU83yHs Carbon 14 dating 1 Carbon-14 forms in the atmosphere through cosmic rays interacting with nitrogen-14, creating a small fraction of carbon-14 in living organisms. When an organism dies, its carbon-14 decays back into nitrogen-14 at a known rate, called a half-life (5,730 years). By measuring the remaining carbon-14 in fossils, scientists can estimate their age, helping us understand the history of life on Earth. https://cdn.kastatic.org/ka-youtube-converted/8wYvKeSK1IY.mp4/8wYvKeSK1IY.mp4 603 Geologic time scale https://www.khanacademy.org/science/biology/history-of-life-on-earth/radiometric-dating/v/carbon-14-dating-2 2020-08-28T13:57:04Z Carbon 14 dating 2 Carbon 14 Dating 2 Sal Khan video https://cdn.kastatic.org/googleusercontent/r54m14ppocGPS9XS0Uh_zYCiXTmBBN7hIGs432HflBuG-Kd8oEcqXwkPpSThY3Yb1yQk0-2e1bz0DwlBL0ukpsHO Carbon 14 dating 2 Carbon 14 Dating 2 https://cdn.kastatic.org/ka-youtube-converted/4YUtnod-YuE.mp4/4YUtnod-YuE.mp4 255 Geologic time scale https://www.khanacademy.org/science/shs-earth-and-life-science/x901efce4a07b98a5:unit-1/x901efce4a07b98a5:history-of-the-earth/v/bhp-intro-geologic-timechart 2023-07-02T10:05:56.724382666Z WATCH: Introduction to the Geologic Time Chart Earth's 4.6 billion-year history has distinct periods. Learn about the four eons - Hadean, Archean, Proterozoic, and Phanerozoic - and how they mark different stages of Earth's history. Discover how geologists use these periods to understand Earth's past and present. Big History Project video https://cdn.kastatic.org/googleusercontent/xcNovJu2Hw_zqtqx_viqLupItyIqezpSjeb_znTvoj0i-AzwWdtIS7sKQfiYKnml2efz4wXqTu3pg1uCUWWmDHaZ WATCH: Introduction to the Geologic Time Chart Earth's 4.6 billion-year history has distinct periods. Learn about the four eons - Hadean, Archean, Proterozoic, and Phanerozoic - and how they mark different stages of Earth's history. Discover how geologists use these periods to understand Earth's past and present. https://cdn.kastatic.org/ka-youtube-converted/UZ1YE68HszU.mp4/UZ1YE68HszU.mp4 397 Geologic time scale https://www.khanacademy.org/science/ms-biology/x0c5bb03129646fd6:evolution/x0c5bb03129646fd6:the-fossil-record/v/earth-s-fossil-record 2022-10-27T04:06:42.11408945Z Earth's fossil record The collection of fossils and their placement in chronological order (e.g., through the location of the sedimentary layers in which they are found or through radioactive dating) is known as the fossil record. It documents the existence, diversity, extinction, and change of many life forms throughout the history of life on Earth. Khan Academy video https://cdn.kastatic.org/ka_thumbnails_cache/cc856251-ded7-4aec-ba20-5cac25a401d9_1280_720_base.png Earth's fossil record The collection of fossils and their placement in chronological order (e.g., through the location of the sedimentary layers in which they are found or through radioactive dating) is known as the fossil record. It documents the existence, diversity, extinction, and change of many life forms throughout the history of life on Earth. https://cdn.kastatic.org/ka-youtube-converted/2f38r4qKkLw.mp4/2f38r4qKkLw.mp4 354 Geologic time scale https://www.khanacademy.org/science/ms-biology/x0c5bb03129646fd6:evolution/x0c5bb03129646fd6:the-fossil-record/a/earth-s-fossil-record 2026-04-27T10:30:47.716252349Z Key points: Earth's fossil record Review your understanding of Earth's fossil record in this free article aligned to NGSS standards. article